Korra's flashback shows Toph seemingly getting blood-bent by the fellow on trial, who I'm assuming is Yakone (the man who threatened Republic City 42 years ago). It seems that Tarlok is somehow related Yakone; perhaps as a son or an apprentice.
Amon can lifebend/energybend/spaghettibend/whatever-you-want-to-call-it-bend, and we previously thought that only Aang (or the Avatar) could do that. Now that we've seen Tarlok bloodbend without the requisite full moon (and of course, Toph inventing Metalbending), we know that the rules about bending are never hard-and-fast.
The idea behind bloodbending isn't that complicated. There's water in blood, thus it follows that it can be bent. 80ish years is a long time, and I can't imagine that no other waterbender figured out that you can control someone by controlling the water in their body.