The Legend of Korra

(1) The fire nation village she terrorized did not know she was a Waterbender; they just thought she was some sort of witch.
(2) Hama says she and Katara are the last Southern Tribe Waterbenders (kind of funny since Katara is trained in the Northern Tribe style), so it's likely that her companions are dead.
(3) In the fire nation, I wouldn't reveal that a prisoner escaped. :/ She also could've killed him, I don't remember exactly what she does to him.
(4) Katara only bloodbent one person outside of the Hama incident. Again, he had no clue what was going on.

The key is knowing that there is water in everything. Everyone who was bloodbent in ATLA did not know that the water in themselves was being bent, they either thought there was some magic or were just confused as to what was going on.

Regardless, I find that it only really takes one person from any of those instances and then a few more people to put things together to make a believable case. I mean Zuko saw Katara bloodbend the captain guy. Sure neither of them knew exactly what was happening, but I think you can make an educated guess (and Zuko could probably confirm it with katara later). Turns out also that the guard Hama blod bent was just knocked out, so he could have had stories to tell to- like "KK kids- never screw with a water bender! They can do this funny thing where you can't move at all and they can control your movements!"

Another thing would be lightning bending for fire benders. Granted it was "common" enough that only "strong" fire benders would know how to do it, as seen in Legend of Korra, it's a common place skillset now as they supply electricity to manufacturing plants and what not. That bending was reserved for only the "strongest" benders, yet it's such a common thing now. Naturally it became common knowledge and as such, so could blood bending.

A little more farfetched, but it could also have been documented in a book somewhere in a library. I mean things back in the day were recorded that way most likely and I'm sure some people went out of their way to document everything. I just find it hard to believe that Hama was the absolute first and only person to think and conceive of bloodbending especially when all it took was knowing the fact that all life needs water :/