They were created in response to police killing blacks on a regular basis. While there are black supremacists and black nationalists in the ranks, the purpose of the organization is not inherently supremacist.
I won't deny that there might be some systemic racism against blacks still, but I highly doubt the police systematically targets blacks. Cities with black mayors and black police officers target black people just as much as cities with white mayors and white cops.
The single biggest predictor of delinquency is single motherhood, which is extremely prevalent among black Americans. And it's laws passed by Democrats (war against poverty) that highly incentivizes black mothers to kick their partners out and get government handouts. This is probably the reason why blacks are so overrepresented in crime statistics.
Regarding BLM: there may be some individuals with noble goals among them, but the leadership of the movement is full of fraudsters. The progressive media does its best to emphasize the craziest ideas among the movement, weeding out the moderate voices. BLM is already beyond recovery.