As a black man here's my feelings on the subject. Resident evil 5's early trailers did strike me as racially insensitive. Left4dead 2 however, does not.
The problem with RE5 wasn't that the zombies were black, but rather that the protagonist was white. There's an underlying theme that was being portrayed, that of a white man coming to save the crazed Negroes that are too incompetent to deal with the situation themselves. This theme is very popular throughout the varying entertainment mediums. For example, there are many movies centered around high schools in the ghetto/hood. The school is usually filled with minority kids and there's gang violence, teenage pregnancy, all the teachers have given up yada, yada. Then what happens? Some white teacher/principle transfers over, is appalled by the conditions and low expectations and they decide to change it. Some how, almost miraculously they do what none of the black faculty could do, they inspire the kids and the community. This is what I like to call the “White man came down the mountain” theme, as the white man descends to the lowly level of the n**** to help uplift him from his own incompetence. With the prevalence of this theme in our culture, it's no surprise that many viewed the early trailers for RE5 (which portrayed Chris Redfield as a beacon of hope in a savage land) as a bit racist. However, I must say the final product was far less offensive. The addition of Sheva and the black special forces troops, gave us proof that Africans can and will do for themselves.
I don't really get any racial vibes from L4D2 and that might be due to the fact that it's being developed by a company I trust are sensitive to the subject matter they are dealing with. I mean you can't pick New Orleans without knowing you have to tread carefully. After the cluster-fuck that was Katrina response, I think that using New Orleans as the backdrop for a game called "Left 4 dead", is as apropos as one can get.