Isn't that a straightforward question? Self-interest turns into racism when the candidate voted for is not the best candidate. If you vote for an inferior candidate on the basis of race, then you are not acting on your own self-interest. I use the terms "best" and "inferior" to refer to the candidate's benefits to the specific voter.
But one can make a similar self-interest argument for white people voting based on race also.
The question is when does self-interest turn into racism and whether the notion of racism even applies to voting.