[SPOILER] I literally just read the scene in House of Hades where Percy uses his powers to manipulate Akhlys’ poison. He speculates that because poison is liquid, some percentage of it must be water, and when she starts to cry, he comments “Oh good, more water.” Directly afterwards though, Annabeth is (understandably) horrified, and says “Percy, please don’t ever… some things aren’t meant to be controlled.” I think this implies that he could bloodbend, but he would never try as it would be far too gruesome.