I mean, there is a difference between pledging support to the Cuban people and pledging support to the Cuban regime. The BLM letter clearly does the former.
This isn't to deny the government's treatment of its citizens; but I feel like a lot of conservatives are going out of their way to paint the statement as hypocritical because it apologizes for human rights violations in Cuba. I don't read it that way. And it is true that geopolitical treatment of Cuba by the liberal West has been a factor in their current economic situation.
Communism may or may not be a viable national system, but in the case of Cuba it's impossible to really tell since the U.S. and other superpowers have meddled in their affairs since the '60s.