Blade is a relatively recent addition to the Avengers. To gain acceptance into their ranks, the vampire hunter had to initially aid the super-team in disguise, most notably as Ronin. It seems that teams like the Avengers have difficulty trusting Blade. That very suspicion could make for a great sub-plot in an Avengers film - it's always fun to watch people who don't like each other being forced to work together. Why he won't be in the Avengers movies: For once, it's not because of legal nonsense - Marvel Studios got the rights to Blade back and even have a movie script in development. I almost hope they ask Wesley Snipes back if the project gets green lit. But don't expect Blade to join the Avengers on the big screen - his dark, vampire-strewn milieu is too aesthetically different from the squeaky clean vibe the MCU is going for. Sadly, the films just aren't as eclectic as the comics. Heck, it'll be tough enough to get that other supernatural-themed character, Doctor Strange, into their roster, and he's a white male.