Auggie, you know how often I agree with you, but I would find an African-American Phyllis perplexing, even if played by a lady as glamorous as Vanessa Williams.
There are numerous references to the idea that Ben married Phyllis because she was better suited to his social climbing than Sally. Even Young Phyllis acknowledges as much: "Oh, I know I'm not much now, Ben..." but I can learn, even if it means walking every museum in town until my feet bleed. Etc. and so forth.
It's hard for me to understand how marrying a black woman in the 1940s (an act that would have been illegal in many, if not most states) would have aided Ben in his determination to rise socially.
Of course, by the end we learn that Ben probably married Phyllis because she was the one he really loved (Sally is merely the one with whom he feels most comfortable), but that doesn't change the lie he has been telling himself for 30 years. That lie doesn't make much sense if he does something "radical" like marry outside his race.
In all other aspects, I agree that Williams could make a sensational Phyllis. But this is one of those cases where race matters. Of course one might also cast a black Ben. I'd have to think about that one, but it would certainly make Ben very sympathetic!