Dr. King was a socialist. If you read his book, he was anti-miltiary, pro-stimulus checks (UBI), and fought against capitalism. He alienated a lot of his followers and had to be careful when supporting those ideas. He was shot not for fighting for Civil Rights, but for fighting against capitalism.
BLM are not Marxists. Marxism is a fight for economics. BLM are fighting for Civil Rights legislation.
On that note, Jesus Christ was also a socialist. He didn't recognize kings or rulers, saying the poor's quality of life mattered more. That's why God always says to feed the homeless and feed the poor; that's one aspect of socialism.