^I wasn't aware of Glover having played Marlowe. But is his take on the character as well known as Easy Rawlins? Will it ever be more than an interesting one-off take?
Shaft, as you say, is a take
on 007 and Hammer. Would simply taking either character and simply having a black actor play them have been as successful? No. Instead, by making an original, standalone black character, an iconic character was created (not to mention a bloody great theme song).
The point about Blade v Van Helsing is that Marv Wolfman and his co-creator (memory fails me at the min) is that they created a memorable and original vampire killer and happened to make him black. They didn't go, 'hmmm, who's an interesting vampire killer? Let's take him and have him played by a black actor, 'cos it's time he was played by one.' They made their own. And he's at least as famous as VH at this stage.
As regards the last paragraph of your post, I'm afraid I simply don't understand it. I haven't advocated making any Asian or black characters white.