I disagree with your analysis for many reasons.
1. Consider the reason that there "are many more black Democrats than Republicans." It is obviously not your position that this is a coincidence, so your position is either:
a) black people are too stupid to know what's good for them;
b) black people are too stupid to see through the Democrat's propaganda;
c) the Republicans are alienating black people intentionally;
d) the Republicans are alienating black people through indifference;
e) the Republicans are alienating black people through incompetence;
f) the Republicans are alienating black people maliciously;
g) the Democrats are doing a better job at courting black people;
h) black people have determined that the Democrats are the lesser of two evils from their perspective.
So which one is it? Some? All? And I am asking you... not "some people"... Notice (assuming a black Republican would need black voters' support) that all the reasons are all indicative of a political environment that makes it unlikely that the Republicans would nominate someone black for POTUS.
2. Obama is conclusive. Obama's nomination and subsequent election is conclusive, irrefutable proof that the Democrats are capable of nominating and electing a black POTUS. No such proof exists for the Republicans. Those are the cold hard facts. I realize it is inconvenient to your worldview, but that isn't relevant to the truth of the matter.
3. Your claim that "Condaleezza Rice would do well" is rank speculation. Wishful thinking at best, misdirection at worst. The fact is Condaleezza Rice has never run for POTUS. There is a reason for this, and the reason is not that she shies away from the White House or national politics. I think that her record indicates that she has an appetite for high office, because she has served at the highest levels of government. The fact that she has not run suggests to me that she is skeptical that she would stand any chance of winning. I share that skepticism for the following reasons:
a. She is female. I am skeptical after this last race that America would elect a female POTUS, and Condi may share this skepticism.
b. There is substantial proof that the Republicans would not nominate a black candidate, as they have never done so despite numerous opportunities.
c. I am skeptical that black Democrats would support a black Republican candidate, and I am skeptical that Condi could win without a majority of black voters supporting her.
d. I am skeptical that America would elect an unmarried candidate as it has only happened twice, and both times were very long ago, before television. I think the optics of an unmarried candidate would be impossible to overcome, the speculation of homosexuality that would result alone would be fatal to a Republican candidate.