Powell would win and would have everyone behind him without a peep, but he wouldn't run. He didn't want to. It's popularly said his wife refused to let him, but that's not the reason.
Anyone before that would have to be in the past. The modern Republican party is not a party helpful or dedicated or whatever to black Americans, and a black Republican is like a gay Republican both in similar prospect of being a minority in the party as well as the party not paying attention to their interests and often going against their group's interests.
In the past, when there were Liberal and Moderate Republicans, and not just as some minority but as a dominating force, that was certainly not the case; there were many black Republicans. The Democrats may have had Roosevelt, but the Republicans were still the party of Lincoln. There was all the more reason since the people around them that abused them were Dixiecrats if they were Southern; the Conservative faction of the Democratic party which later broke away and joined the Republican party and became it's base*. This was an era when Martin Luther King was a Republican (he left it because Goldwater won the nomination. King was a Liberal Republican, despite what the modern universally Conservative Republican party may try to claim.) and you had elected black politicians like Edward Brooke.
If a black Republican were elected in the era of the Rockefeller Republican, which is the last time such a thing could occur, it'd be a totally different animal. Dixiecrats would be repulsed, the Northern Democrats would have to contain the fanatic repulsion of the Dixiecrats (that should be fun), elements that had supported Goldwater (not all, but an amount of them) wouldn't like it and the Eastern Establishment Republicans would be perfectly fine with it since he's one of their numbers and ideologically they have no issue with him (as with the Northern Democrats). In short, the Liberal elements would be fine with it by and large and the Conservative elements by and large would not be, certainly not the Dixiecrats.
*I make special mention of that because I'm tired of hearing "Well Democrats [Insert Complaint]" when in actuality, those Democrats joined the Republicans and are often the forefather of the critic. And I'm also sick of hearing "Well the Republicans did [Insert Good Thing]" when the Republicans that did that thing left the Republicans and became Democrats. It's especially odd hearing Black Republicans say that about African Americans and their treatment by the Democrats and Republicans, because the people that did things positively for them are no longer in the party, nor are their beliefs part of the party, and the descendents of the faction that condemned them and did all sorts of horrid things are now their fellows in the party. It shows a total ignorance of history.
Past Black individuals who were Republicans were also not Conservative Republicans but Liberal or Moderate Republicans at total odds with what the Republican party is now, and who generally, if they lived to see the Republican party evolve into a totally Conservative party, left it. Such was the case with Martin Luther King, who was repulsed by Goldwater and his Conservatism and supported Lyndon Johnson in 1964. You can't just have a name on a thing totally different from its predecessor but with the same name and then act like it was the same thing whatsoever. It'd be liked the United States of America trying to claim the glory and achievements of the Roman Empire as something it did, and saying that Trajan was a great American patriot.