Fans were ecstatic when it was announced in 2019 that Blade was officially joining the Marvel Cinematic Universe played by actor Mahershala Ali, but the Daywalker was almost part of another cinematic universe several years before that. In fact, if the creators of the Underworld franchise had their way, audiences would have seen Blade battling vampires and werewolves alongside series star Kate Beckinsale in a crossover film.
While promoting Underworld: Blood Wars at New York City Comic-Con in 2016, Beckinsale revealed during a Q&A that the creators of the long-running horror/action franchise actually had an idea to include Blade in a future Underworld film and had reached out to Marvel about a potential crossover. Sadly, Marvel declined, stating that they had plans for the character.
The Underworld franchise started in 2003 and follows the exploits of Selene, a vampire who finds herself caught in the middle of a war between her kind and werewolves, or Lycans as they are referred to in the films. Meanwhile, Blade, released in 1998, was the film that kicked off Marvel's current dominance at the box-office. Played by Wesley Snipes, Blade appeared in three films chronicling the half-human/half-vampire's attempts to wipe-out fellow bloodsuckers. The final film in the franchise, Blade: Trinity came out in 2004 and was followed by a short-lived TV series in 2006, with rapper Sticky Fingaz taking over the role.
Not much is known about the potential crossover between the two franchises beyond what Beckinsale said at that Comic-Con. Though the original Blade franchise never mentions a war between vampires and Lycans, the events of the Underworld franchise could have taken place after the Blade films, especially since later entries in the series do jump ahead in time. It is also unknown if Wesley Snipes would have returned to play the part, but it's possible that the creators of Underworld intended to introduce their own version of the Daywalker played by a new actor with no connection to the previous incarnation of the character.
At the time of Beckinsale's quote, it was unclear as to what exactly Marvel's plans were for Blade. Three years later, Marvel officially announced a new Blade movie with Mahershala Ali taking over the role, though as of this writing, the film still doesn't have a release date. Unfortunately, it seems unlikely that the crossover with the Underworld films would have happened, even if Marvel allowed the producers to use Blade, as Underworld: Blood Wars had the lowest box office in the franchise and Beckinsale ruled out appearing in future sequels. An Underworld TV show was also in development at one point, but it's unknown if that will still happen.
A crossover with the Underworld franchise could have been a fun and interesting way to bring Blade back to the big-screen, but Marvel's long term plans for the character prevented it from ever becoming a reality. Perhaps one day the producers of the Underworld films will reveal more about their plans to include Marvel's vampire hunting Avenger, but for now the possibility of Blade fighting vampires and werewolves alongside Selene remains nothing more than a footnote in the history of the character.