I think that would mean war. I don't think the US can block ships with food, fuel and medical aid. They can block ammo, guns and guns parts (the DPRK used to sent guns to Cuba during the 1990's - 2000's but they US didn't like that the DPRK was selling guns to Latin American countries like Peru, Ecuador and Colombia). All they will probably do is mark these ships as "breaking the embargo" and refuse to let them sail on US owned ports (Russia shouldn't care, they already are banned from US ports).
Is there a way for the US to block BRICS aid?
I think that would mean war. I don't think the US can block ships with food, fuel and medical aid. They can block ammo, guns and guns parts (the DPRK used to sent guns to Cuba during the 1990's - 2000's but they US didn't like that the DPRK was selling guns to Latin American countries like Peru, Ecuador and Colombia). All they will probably do is mark these ships as "breaking the embargo" and refuse to let them sail on US owned ports (Russia shouldn't care, they already are banned from US ports).
BRICS as a whole, if they trade with Cuba, must have decided to call the US’s bluff because the US needs BRICS more than BRICS needs the US.
The US is not going to actually stop trading with BRICS, that would be utterly self destructive