I think two of the biggest causes of crime are poverty and living in a densely populated area. Blacks are disproportionately poor and disproportionately live in densely populated cities. Therefore the cause is not them being black but rather where they live and what their income is.
They overwhelming vote democratic because most of the people with animosity towards blacks are part of the republican party. They find a strong value in community that supports each other and they feel the republican party is more about individuals. I don't see how this contributes towards crime.