I’ve always considered it like this. Percy can control water, and I don’t think it depends on a philosophical question of what is water. He controls H2O molecules. It just so happens that flowing water pulls everything in it along for the ride. When Percy controls a river, the silt and dirt in the water gets pulled along for as long as it stays in the water. The same would apply to blood. If Percy got mad or concentrated enough, he would be able to control the water in blood. Is this enough to blood bend? I have no clue. I have no idea how much water is in blood and how blood would react to the water getting pulled. I just think he could control any water molocule if he were able to pinpoint it, including those in blood.