Indeed, one of the rules of BRICS is that countries cannot sanction one another. This would open Cuba to unfettered trade with the biggest economic bloc in the world.
The actual mechanisms of the Cuban blockade are that any ship/shipping company that trades with Cuba is banned from American ports for the next 6 months. If BRICS is openly inviting Cuba in, that means that a meaningful chunk of the economic bloc's logistics companies feel comfortable effectively never dealing with the US, which has massive implications for global trade going forward.
Indeed, and I think this is possible because BRICS is already a bigger trading bloc than G7. It's also where most actual manufacturing happens. So, if the US continues its policy then US companies will end up in far more pain than the respective BRICS companies. Incidentally, it's the same dynamic that we currently see playing out with sanctions on Russia where most of the pain is being felt in Europe.
I don't know enough about Chinese business to be sure, but theoretically they could make "The Sino-Cuba Shipping Company" that handles all direct trade with Cuba and could sideline that rule.
It's my understanding that the US blockade bans a ship that has docked in Cuba from docking at a US port for 6 months after. So it's not an all or nothing matter for a country doing business with Cuba. It's still a big disincentive for a shipping company.