If you have demands from the democratic party, you don't "not vote" or you'll get nothing at all (look at how Bernie and his platform were ignored after he didn't win against Hillary).
If you have demands, you show up at the smaller state elections/midterms and vote for the democratic candidate who best represents your demands. Once they get into office, if they aren't serving black people enough, you show up at the next election and rally behind the next democrat who is. This will 1. deliver the message on your demands and show the numbers behind them 2. exercise your right to choose, as you have in a democracy.
There are a lot more elections than the democrat vs republican presidential ones.
How many of you voted in your state's primaries the past few months and voted for the most progressive democrats?