Allegedly 1.5% of open source software contributors are female. So you are more likely to win the lottery than finding yourself working with a black woman on an open source project.
I would expect that the two aren't independent -- the proportion of black women to black men could well be very much higher than the proportion of white women to white men. The same qualities and situation that help a black man become a programmer despite the various forms of systemic bias would also be useful to a woman.