In DPOS 51% of the stake does not give a stakeholder control of the blockchain. In order to take control you need 2/3 plus one of the top witness positions. The 51% figure only applies to POW or traditional POS.
This is all in theory since in practice voter apathy creates a situation where not all of the stake votes on block producers and you need much less stake to take control. If it wheren't for that soft fork 0.22.2 would not have been necessary since steemit only has ~30% of the staked steem.