Nothing wrong with that idea except that I rather not have Sony getting Blade, hell I rather Sony give up Spider-Man one of these days, see what Marvel Studios could do with him.
I know that Marvel Studios won't make R rated films, but as we can see from IM and TIH they don't shy away from death or violence, so I definitely think they could handle Blade. (be tough to top Snipes though)
They don't exist in two entirely different worlds, they are both part of the Marvel Universe, vampires is just one of the billions of threats present in it, and one of the better concealed ones at that.
Some people might think that bringing these two properties together would be a disaster, downright wrong, all muddled up with no clear direction... And it could be if it was handled by idiots...
One of the most important things to kept in mind is that it not be handled like Versus or Team-Up movies are (basically cheaply made or at least creatively poor and simply banking off of a gimmick.)
If Sony, Fox or anyone else did it I'd puke.
On the other hand if Marvel Studio was in charge I'd see this as a brave new step in superhero movies.
A superhero movie that starts off with an established universe and established characters, no origin story;
instead the movie focuses on telling a story, a story in which two unlikely partners need to work together, while Blade is a veteran at tracking and killing down Vamps of all sorts, he's never encountered one like Morbius and Spidey has reason to stop Blade from killing him and trying to get Blade to play by his rules even though Blade is convinced that it is his game.
Marvel isn't ready to do something that audacious yet, even if they were able.
The first step in this direction is the upcoming Avengers movie - by the time it comes out we will have been made familiar with all the heroes and possibly the villain (if the story revolves around fighting Hulk, seeing the good in him in light of a common enemy The Leader).
