From an INDIEWIRE article about Kerry Washington hosting Saturday's episode of SNL (I only watched the first half-hour):
Perhaps more revealing than the complexion of the show's cast was the fact that virtually all the characters Washington was given to play, from a Spellman poli-sci prof to a teacher at Booker T. Washington high, was written as black, which is to say the writers started out saying not, "What can we write for this beautiful, prodigiously talented actresss?" but "What black characters can we write for her?" The lack of voices goes beyond what public characters the show can and can't impersonate; it affects what it can, and can't, imagine.
Link to the complete article: http://blogs.indiewire.com/criticwire/kerry-washington-snl-race
For some historical perspective, let's look at a tv.com comment about Season 5 (the post-Belushi/Aykroyd year) of SNL, when Garrett Morris was the only African-American cast member:
"The Incredible Man," a [parody] of The Wizard of Oz, marks the nadir of the show's mistreatment of Garrett Morris. After being put through several women's parts, Garrett was cast as a flying monkey for this sketch. Six day[s] later, Garrett broke down in front of the entire staff, no longer able to contain his resentment for five years' mistreatment.
Link to article plus comments: http://www.tv.com/shows/saturday-night-live/elliott-gould-gary-numan-116398/