He probably would not have sold as much. He would have probably never been a part of Motown Records, which was the only real rival to the Beatles in the 60s and the only real rival to the major labels in the 70s.
Remember, Motown as a whole (and their unrecognized musical backbone the Funk Brothers) had more number ones than The Beatles, Elvis Presley and the Beach Boys combined. Motown was a significant and an almost-wholly white force. Sure, there was the Rare Earth imprint (named for the band Rare Earth, who had several excellent songs), but for the most part, Motown was an all-black endeavor.
And Stevie Wonder benefited from this. Considering his race, his disability and the time when he first became popular... he was lucky to have the success he did. This is not to say he wasn't a brilliant musician. He had perhaps the greatest run of albums in a decade than any artist outside The Beatles.
In the 1970s, he released Music of My Mind (1972), Talking Book (1972), Innervisions (1973), Fulfillingness' First Finale (1974) and Songs in the Key of Life (1976). If he were not on Motown, he probably would not have not released these five albums. If he were not black, he would probably have never been on Motown Records. So, in my opinion, if Stevie Wonder were not black, he would have had less success.
Being blind was only an additional marketing tool for a musician as brilliant as Stevie Wonder.