Has anyone ever gained control of the blockchain? I've heard discussion that if you have 51% of the network's hash power, you can effectively accept or reject any changes to the blockchain (and therefore create undeserved gain for yourself).

Has anyone ever accomplished this? It's theoretically possible, but has it been done?
I think it is safe to assume that the creator once had full control of the blockchain. And at the beginning, there probably were times when someone had 51% hash power.

However, those people would have no incentive to perform an attack, so my guess would be that it has not happened yet.