Why would they need to do it secretly. They can JUST DO IT.

NO THEY DON'T.

51% of the hashes only gets you 51% of the blocks. Alice can submit transactions that stay in the mempool and get accepted on 49% of the blocks. There is absolutely NO censorship in this case. It doesn't even inconvenience Alice because her transaction can stay in the mempool for hundreds of blocks. Hive account@dan fails.

To conduct a 51% attack, Hive account@dan has to mine secret blocks until he has a longer chain then force a chain reorganization by publishing his fork. While he is trying to get a longer chain, Alice can push a transaction through to bittrex because Hive account@dan is going to lose a 2 block race many times over the course of a day. That's how Hive account@dan fails.