I went to undergrad in Boston and I and my other black friends always went out at night. Now, if you're in southie at night, that's another story. Although in the main parts of the city you won't see very many black people, it's not unsafe. Cambridge is a little better but not much. From my experience in San Francisco, there weren't many black people around either, but I felt more comfortable as a black person there than I do in Boston. That said, there are many black graduate and undergraduate students in Boston because there are so many universities there. The good thing about school is you have a ready-made community so to speak. If you like BC I would go ahead and apply, see what happens, and then visit.
p.s. How did your LSAT retake go?