Yeah, it should. If the people marketing the movie were smart they'd be targeting general audience which includes people who haven't seen Blade 1 or 2. The poster gives no hint what it may be about. All we know is its called "Blade: Trinity".
Ever since Matrix Reloaded started the whole "close-up of a character with a reflection in his sunglasses" motif with its posters, everyone has been doing it.
It's the third one. Come on, if someone doesn't know he's a half man half vampire he either:
a) didn't pay any attention to the plot of the first two movies
or
b) didn't watch the first two, even though 1 has been on TV, and 2 is on the movie networks. If they haven't bothered to see either of the other movies, what are the chances they care about this one?