Harry realized the man was using his hardened blood as crystals to defend. It gave him the idea to use blood as an extension of his body. Until now, he had just been using blood as a weapon that would injure others if they had open wounds.
There was a reason he had never realized this kind of use for blood.
He couldn't control the blood inside other people's bodies, but he had forgotten that he could control his own and use it as an extension of himself.
This discovery opened a whole new world for him to explore. The obvious drawback was that when blood comes out, it needs an exit, and that would leave holes in his skin.
But did he care? The biggest reason to avoid skin tearing was blood loss. But he could control his blood to prevent it from leaking, and his blood regeneration speed was far faster than that of any normal being.
