The following contains spoilers for Black Widow.
Superhero movies have developed a reputation for including Easter eggs for long-time fans of the properties. Sometimes, they're nods to the comic book properties that inspire them, while others, they're nods to the movies that come before them. In the case of Black Widow, both types of Easter eggs are peppered throughout the movie.
Black Widow allows for a prequel of sorts for the title character. Set during the events of Captain America: Civil War, the title character returns to her past to help the family formed for an undercover mission when she was still a child in the Red Room. While the story explores her past and gives her some closure, it also gives the fans a chance to search for a whole lot of Easter eggs.
SHIELD And WHiH
When Alexei Shostakov gets home for dinner in Ohio in 1995, it's not a normal homecoming. He informs his family that they're going on an adventure. The adventure, as it turns out, is returning to Russia because they're a family of spies. He knows that someone will be after them, so he takes his family and heads to a plane. Alexei isn't wrong that someone is after him. The vehicles that pursue the family aren't regular law enforcement. Instead, they're marked with SHIELD logos. SHIELD might not be active in the movies in the modern day, but it was at its prime in the '90s.
Likewise, when the family gets away, a news broadcast is seen detailing the spies that hid amongst regular citizens. That broadcast is by WHiH25. WHiH is the unofficial news network of the MCU. It's appeared in several movies and television series, and even been used to promote the release of Ant-Man.
1995 In The MCU
It has to be pointed out that 1995 is a rough year for women in the MCU - and a very busy one at that. Based on the timeline of the movies so far, there is a lot that goes down other than Natasha and Yelena ending up back in the Red Room for training as children.
In 1995, Captain America: The Winter Soldier reveals that Nick Fury saved Alexander Pierce's daughter from a hostage situation. Fury was busy since Captain Marvel is also estimated to have taken place the same year. Likewise, Ava Starr of Ant-Man and the Wasp ended up a "ghost" and orphaned thanks to a lab accident. That's also the estimated year for when Thanos slaughtered Gamora's people and took her to raise himself.
The Sokovia Accords And The Airport Brawl
When General Ross pursues Black Widow, he notes that she's in violation of the Sokovia Accords for assaulting the king of Wakanda. While it's not exactly a traditional hidden Easter egg, it does give the audience an idea of when the movie is set. With General Ross' assertion that Steve Rogers is still on the run, but Hawkeye, Ant-Man, and more have been captured, it places Black Widow right near the end of the events of Captain America: Civil War.
There's another Civil War reference in the movie as well. When Dreykov gives the okay to activate the Taskmaster protocol, Taskmaster is reviewing fight footage of the Avengers in action. Specifically, Taskmaster is watching Hawkeye's fights at the airport during Civil War. The footage gives fans a hint of what's to come as Taskmaster uses the fighting styles of most of the Avengers throughout the movie.
Yelena's Knife Work
When Yelena is first introduced in the movie, she's the leader of a team of Widows going after a target. The target, however, spots Yelena and the others, and tries to escape. That's likely because the target is a former Widow herself and knows what to look out for. When she and Yelena get into a fight on the streets of Morocco, however, the audience might notice some familiar movements.
Yelena drops her knife from one hand that is immobile into the waiting fingers of the other hand, allowing her free range to use it against her target. That same move is employed by the Winter Soldier. It's either a common trick amongst assassins, or it's a subtle callback to the character since the same technology that brainwashed him is what's used to control the Widows as well.
Mason
Natasha's go-to person for a little illegal help is Rick Mason. While he doesn't get much screen time beyond trading quips with Natasha, Mason is inspired by a comic book character. In the comics, Rick Mason is also known as The Agent. The son of a supervillain, he works as a mercenary of sorts, performing illegal tasks for the highest paying offers. His line of work in Black Widow isn't all that different.
Tank #409
When Yelena and Natasha are pursued in Budapest (and the audience finally learns what Hawkeye and Black Widow were up to there years earlier), the tank Taskmaster drives has a #409 on the front. Numbers are tricky when it comes to Easter eggs because they could easily just be a coincidence. This one, however, might have some meaning.
In 2009, a Black Widow series called Deadly Origin was published by Marvel comics. Its fourth issue featured Natasha running headlong into problems from her past as she tried to uncover the truth about the Icepick Protocols. Now, in that storyline, Natasha ended up in space, not back in Russia, but the themes of confronting her past remain.
Though he's not credited, Hawkeye also has a slight cameo for the Budapest storyline as well. What sounds like Jeremy Renner's voice comes over the comms in the Budapest flashback.
The Black Widow Landing
Deadpool making fun of superhero landings made audiences pay a lot more attention to just what a superhero looks like when they manage to catch themselves. Black Widow, however, has a very distinct landing pose that involves one of legs being extended. It's a pose that's been used in the MCU since Iron Man 2, and that's partly because it's a pose drawn in the comics. Yelena Belova, however, thinks it makes Natasha look ridiculous and calls her out on it.
After Yelena comments on the pose, however, that's not the end of it. Natasha and Yelena both land in the pose at different points in the movie, but they aren't the only ones. Even Milena Vostokoff lands in the pose during a fight scene as well. It becomes a bit of a running gag for the family of Widows.
Alexei Calls An Inmate Ursa
When the audience is reintroduced to the Red Guardian in the modern-day, he's in prison. Alexei happens to be arm wrestling other inmates while getting another tattoo on his back. One of the inmates he arm wrestles, he calls Ursa.
Not only does Ursa mean "bear," but it's also potentially a reference to a Marvel comic book character. Mikhail Ursus uses the codename Ursa Major in the comics. That's the name of a constellation for those who aren't astronomy buffs, and it's an appropriate one, because it means "big bear." Mikhail is a mutant who can turn into a bear.
In the same scene, the audience should pay attention to Alexei's tattoos. While many of his tattoos are based on research into real tattoos created in Russian prisons, there is one that's specific to the movie. Alexei has a portrait of Melina near his neck.
The Widows Ledgers Are Dripping In Red
One thing that's made abundantly clear about Alexei is that he has trouble reading the room when it comes to the daughters he adopted in his spy family. He goes on about being so proud of their kills counts, referencing how their ledgers must just be dripping red.
MCU fans will remember the red ledger from Natasha's conversation with Loki in The Avengers. She wants to wipe it out, but Alexei doesn't understand her guilt. Of course, there's also another payoff to Natasha's conversation with Loki in Black Widow. During that same talk, Loki is the one who first mentions Dreykov's daughter in reference to Natasha's guilt. The audience finally gets to see who Antonia Dreykov is in Black Widow.
The Iron Maiden
Melina tells Natasha that she cycled through the Red Room four times before Natasha was even born. That's a lot of training to become a Widow. In the comics, however, Melina isn't a Widow. Instead, she's the Iron Maiden, and there is potentially a small reference to that in the movie.
When she and Natasha talk in her storage space full of weapons, the masks that the women use to temporarily swap identities can be seen on metallic looking heads in the background. Those look suspiciously like the mask Melina dons as the Iron Maiden in the comics. The masks in the movie, of course, are the same pieces of technology that Natasha uses to pretend to be someone else in Captain America: The Winter Soldier. Agents Of SHIELD and The Falcon and the Winter Soldier have also used the technology since its debut in the MCU.
The Crimson Dynamo
While the audience knows that Alexei is the Red Guardian, Yelena also references another name when speaking to him. Though he does correct her mistake, she calls him the Crimson Dynamo instead.
While the Red Guardian is the Soviet answer to Captain America, the Crimson Dynamo is the Soviet answer to Iron Man. It's no wonder Alexei is a little insulted at Yelena's slip. They probably have a similar rivalry in their potential MCU backstory. Of course, one of the people to use the moniker in the comics is actually Anton Vanko, who was referenced in Iron Man 2, the same movie in which Natasha made her MCU debut.
Thank You For Your Cooperation
After her talk with Dreykov in his office, Natasha says this particular line to him. If it sounds familiar, it should. She offers the same line to Loki in The Avengers when he doesn't realize she's using her own perceived vulnerabilities to interrogate him.
In fact, Natasha's entire sequence with Dreykov, allowing him to think he's got the upper hand and gleaning all of the information she needs from him, is right out of her opening scene in The Avengers as well. There, the criminals that kidnapped her didn't realize she was just biding her time to get information.
Yelena's Vest
It hardly seems fair to call this an Easter egg since Yelena's vest is a running talking point throughout the movie, but it is worth a mention. The vest that Yelena is so proud of was last seen on screen in Avengers: Infinity War.
It's the same vest Natasha wears while on the run with some of the Avengers. That's because Yelena gifts it to her at the end of Black Widow, a piece of practical clothing shared between sisters who happen to be accomplished spies and assassins.
The Post Credit Scene
The scene in which Yelena visits Natasha's grave features a cameo from Valentina Allegra de Fontaine. Audiences last saw her in The Falcon And The Winter Soldier. Her appearance sets up Yelena appearing in the Hawkeye series on Disney Plus, but that's not the only reference in that sequence.
When Yelena first arrives at the cemetery, she calls a dog to get out of her car with her. The dog's name is Fanny. Fanny was also the name Mason gave Natasha on her fake ID at the beginning of the movie.