There was a time when it would have been very possible for the Blade franchise and the Underworld franchise to crossover with each other, because they were both set up at New Line Cinema. But Blade is a character from Marvel Comics, and Marvel regained the rights to him in 2011. That didn't stop the filmmakers behind the Underworld films from pitching the idea of a crossover to Marvel – and Underworld franchise star Kate Beckinsale would still like to see that movie happen.
Speaking with Variety, Beckinsale – who played the vampire "Death Dealer" named Selene in the Underworld films – said,
I really wanted them to do an Underworld-Blade mashup. What a duo (Selene and Blade) would be. I would definitely do that, but I think they just wanted to reboot Blade as Blade, so they didn’t go for it."
Beckinsale previously talked about this dream crossover at New York Comic Con back in 2016, saying the Underworld filmmakers "had that idea", but Marvel shot it down because "they're doing something with Blade".
Soon after, Marvel Studios' Kevin Feige confirmed,
[The Underworld filmmakers] did ask a long time ago and I think our answer was, 'No, we'll do something with Blade at some point.' That's still the answer."
Marvel Studios is now working on a Blade reboot that is set to star Mahershala Ali as the title character, with Bassam Tariq is talks to direct the film from a screenplay by Stacy Osei-Kuffour. Production is expected to begin in July of 2022.
Selene will probably never share the screen with Blade, and Beckinsale is "highly doubtful" another Underworld will ever happen at all, but the actress is ready to put her character's leather pants back on if the opportunity arises. She told Variety that she still has a pair and
I know exactly where they are. You never know who you want to impress."
While I'm a fan of the Blade movies, I could never get into the Underworld movies and the way they presented vampires and werewolves, so I'm not too upset to hear that the crossover idea didn't get very far.