It can change the power dynamics in the fanfiction world. He could become a very feared or respected figure, depending on how he wields this power. Also, it might make other characters view him differently, either with awe or with fear. And in terms of the plot, it could open up new storylines like him having to hide this power from some while using it to protect others.
The implications are far - reaching. From a character - development perspective, Percy controlling blood could lead to him becoming more isolated as others may be afraid of him. Socially, it could create a divide between him and his friends or allies. In battles or conflicts, it gives him a unique advantage, but also makes him a prime target. It also might force him to learn about the biological and ethical aspects of blood, which could be a whole new area of growth for his character in the fanfiction.
One implication is the moral dilemma. Percy would have to decide when and how to use such a powerful and invasive ability.