Here's an interesting blog post with links to the main article suggesting that certain black women may suffer from a more aggressive form of breast cancer than white women. This in turn would help to explain the differential survival rates between the two groups.
While its likely that socio-economic factors continue to play a role, this particular study appears to suggest that the larger factor may be a genetic one. The poster that I'm linking to here offers another interesting angle. S/he suggests that in addition to socio-economic and genetic factors one should consider looking at the nature of treatment for these cancers and how it is tailored to a white population.
While interesting to consider this on its own I'm not certain that it's not not just a variant of the two prior explanations. After all, if the treatments administered in identical circumstances don't target blacks as well as whites it's likely due to a) genetics or to some extent b) cultural or socio-economic factors.
Gotta love blanket categories :)