I'm not sure if it's a scam or not, but buying futures of a fork is always risky. A lot of users lost a huge amount of money because they decided to buy segwit2x futures, before the fork even happened. At least bdiamond already forked, but just like bgold they will probably took a while before we can claim or mine any coins. If they are ever released I will try to claim them, specially if they are traded closed to their futures price, but I would never buy bdiamond futures though. Actually I don't think anyone should by things they can get for free.