But if I want to do a song by Charlie Patton or Leadbelly do I have to be what is now called Politically/Musically Correct and sing it in a non "black" voice?

Some of those songs, I just wouldn't sing. We have a session here in a bar primarily frequented by black people, and one of the (white) guys who plays started up a spirited rendition of "Cotton Fields Back Home," which really doesn't sound good coming from a white guy. Likewise, I wouldn't sing "Black Girl." But of the songs I'd choose to learn, I'd make them my own, just like Lead Belly and Charlie Patton did. It's interesting to learn what they did and how they did it, but I'd never go so far as to sing their songs in a 'black' voice, because I'm not black, and it's too easy to be unintentionally insulting when doing that. Ramblin' Jack can get away with it because he sounds exactly like Lead Belly when he wants to, but I wouldn't risk it, myself.