The trope where the "black guy" in a horror movie always gets killed seems to me to be the result of two things. One, we tend to not put black men in lead roles in movies (this has only changed very recently in Hollywood) and usually the leading role is the person who's the only one who's going to live in a horror picture.
So by that logic the "black guy" is necessarily the friend of the lead who will, by dramatic necessity, die. So if you have an ensemble cast in a horror picture, you know "the black guy's gonna get it."
Which is really freaking annoying.
This is, of course, racism. It's not racism that we're going to kill off all the red-shirt minor characters, it's racism because only Will Smith is allowed to be a black man as the lead in a big-budget action picture. Obnoxiously, much of the resistance to having more black people play leads in Hollwood pictures as well as tiny pictures of the likes which we make, comes from overseas buyers who (basically) want big American blondes in their pictures.
A very interesting counter-example to the way African Americans are treated in horror films is the king of all zombie pictures, Night of the Living Dead. Because that picture, even though it killed off the black lead character brutally in the last moments of the movie, is not guilty of racism in casting. The racial weight of the all-white vigilante dudes killing the hero at the very end is very poignant, don't get me wrong. And the ending is dark, dark, dark. But otherwise the "rules" of horror are repudiated by that picture simply because a black man plays the lead. He's the hero. He survives. It's like they killed Captain Kirk at the end of that picture, not some anonymous ensign.
Just the whole notion of casting by ethnicity annoys me to no end. The way we do cast "by ethnicity" is by trying to make sure the different characters in the picture look significantly different from one another so that they're clearly delineated to the audience. You put a couple olive-skinned white guys with dark hair next to one another and one of 'em better grow a mustache or else we're just going to confuse the two of them. Make one of those characters an Asian woman and you're in the clear.