They weren't all murderers and they weren't all blood mages. Orsino is a lousy advocate, it's true, but that does not take away the truth of what he said.
The point wasn't whether or not Orsino was wrong, but rather that he was hardly a credible enough source for someone to take him on his word alone. Especially for a Templar. =)
She was clearly using some form of blood magic, but was that innate magic she was born with or a gift from the demon as part of the power she had been promised?. Her daughter and Sebastian both say she isn't a mage, and no magic was seen or used until the demon was found when they were expanding the foundation. In fact, it's made quite clear the Herriman's are proud of how little magic they have in their family.
She would have had to have been a mage any sort of deal with the demon. Also, she kept everything else hidden from her family, why not this?
It is a case of occam's razor, either she was a mage that hid it or due to some random magical exception lady Harimann was able to deal with a desire demon and just happened to start using magic afterwards. Simplest solution here is that she was always a mage.
The sources here is Sebastion and her daughter expressing that they didn't think she was a mage. Harimann herself doesn't say that she magically got her power because of the deal, it isn't addressed at all during the showdown.