I have watched the first season of The Legend of Korra and as far as I can recall it wasn't made clear whether Amon really used bloodbending to take away bending abilities of the other characters. Korra did say that he used bloodbending but it's not that she can't be wrong. The Avatar Wikia site has had its articles edited to include that bloodbending can be used to take away a person's bending but I can't remember anything happening in the series that confirms this. Or have the creators themselves confirmed this?
IMO, it is possible that the spirits provided Amon with the ability to energybend: evil spirits do exist. Also the next book is named 'Spirits', so what are the chances that this book delves into what really happened when Amon ran away from the Northern Water Tribe? Also, the way Amon takes away bending is very similar to the way Avatar Aang used energybending to take away bending.
The only other thing that I can recall is that Amon's brother, Tarrlok said that he recognized the stance of his brother as his bloodbending stance which I don't think is really evidence as to what he really used: Amon's stance when he takes away someone's bending can be similar to his bloodbending stance because he is so used to the latter.
I don't think this question has been asked before and I will apologize if it has. My main question is that what evidences are there in the series which really concludes that he, in fact used bloodbending?