In American leftism there is a definite divide between black and white.

For example second wave feminism is often thought of as Women seeking entry into the workplace, but at the same time black feminists were trying to leave the workforce and take care of their own kids.

The labor movement has an explicitly racist history. A fact that Capitalists often took advantage of by leveraging black scabs who were often ineligible for union membership. Eugne Debs identified this as a problem with the socialist movement.

I'm not saying that racism is common among today's lefties, just that white lefties are often ignorant of black American life and especially black radical thought and activism.

If you are vexed by Bernie Sanders' struggle with black voters, you're probably not very familiar with this history.