The proof of concept for Marvel's film efforts came in the form of Blade, but the trilogy's quality was a mixed bag. Exhibiting the best and worst of the genre at once would be inexplicable, but three films created by three different directors will inevitably have very different results. The trilogy seems to be a default state for genre film, every franchise seems to comfortably divide itself into threes. The model will almost certainly continue, but the results are as varied ever.
1998's Blade came to the big screen from the mind of director Stephen Norrington. This first big screen outing for the character was an immensely fun ride, relying heavily on the director's strong visuals. At the time, he had only directed one other film, a sci-fi horror called Death Machines. Since, his other best-known project is 2003's ill-fated League of Extraordinary Gentlemen. It was that unfortunate project, mired by studio notes and clashes with Sean Connery, that disillusioned Norrington with the industry. Blade is likely the high point of his career in both success and quality, and aside from some special effects work in the mid-2010s, Norrington has been tragically absent from cinema.
Blade II carried a much greater pedigree, and came with the quality to match. This film came from the masterful mind of Guillermo del Toro, who has gone on to become a hugely beloved filmmaker. He's still putting out out fantastic films today, but at the time, Blade II was his first big introduction to American audiences. The year before, del Toro put out the outstanding Spanish language horror film The Devil's Backbone, which landed him the big Marvel movie job. The second film is almost inarguably the apex of the trilogy. Del Toro's horror background, unique tastes, and love of monsters made the film an absolute joy to behold.
After a good start and a great sequel, the third film came just in time to ruin everything. Long-time series writer David S. Goyer directed Blade: Trinity. The conclusion of the trilogy was seen by many as a shallow rehash of the previous films without any of the narrative sharpness or aesthetic glory. Goyer is a complicated filmmaker, more often a screenwriter than a director. He's a veteran of the superhero genre, but his other directorial efforts don't touch the genre. Mostly supernatural horror or dull action. Goyer co-wrote The Dark Knight trilogy, Batman V. Superman, and Man of Steel. Goyer clearly has the skill to write the films, but when it came time to bring his script to life, he tragically faltered.
The three distinct directorial visions, even with a ton of returning crews, crafted radically different takes on the franchise. To watch all three in a row would feel more like episodes of a monster of the week style TV show than a trilogy of films. Unlike many sequels, the directors felt little need to maintain consistency, instead each pulling in their own directions for better or worse. Looking at the bizarre line graph of quality experienced by the Blade series, one has to wonder whether it would have been improved by letting one director keep the reigns.
Consider The Lord of the Rings trilogy, one of the best-loved adaptations of the modern era and the sole creative vision of Peter Jackson. Jackson's faithful interpretation of the source and beautiful visual storytelling were integral to the trilogy, and putting another director in his role mid-series would likely ruin the flow overall. Jackson even returned for The Hobbit trilogy, ensuring he remained the filmmaker of Tolkien's legacy. Compare that sprawling beloved adaptation of an iconic series of novels to the Harry Potter film series. 8 films brought to screen by four different directors, featuring exactly the kind of wild peaks and valleys to be expected of that.
Of course, this rule is far from hard and fast. Even in Blade, the separate directors brought awesome success as well as failure. Surely the wrong director given full control over a trilogy would only fail consistently, and surely fans would prefer a series to glimpse greatness, if only on occasion. Series like Alien, Terminator, or Predator have similar results to Blade. Wild assortment of directors, wild variance in quality, occasional flashes of brilliance, equally occasional crushing failure. There are also examples of single directors with mixed results; Sam Raimi is responsible for the first two Spider-Man films, but also takes the blame for Spider-Man 3. It's far from airtight, but a trilogy with a single director will inevitably lead to a more consistent experience.
While obviously not the only role that matters, the director is one of the most important creative forces on a film. The Blade films can teach a number of lessons, and one of the most interesting is that maybe a trilogy should be treated more like a single story.