He can manipulate his own blood! Not anyone else's. The most frequent and useful offensive application of this is blood bullets: he can cut his finger open and shoot bullets made of hardened blood. It doesn't have to be a bullet, though. In theory, he can create other objects with his blood (such as a knife, a hairbrush, a screwdriver, or a set of house keys). However, he's using his blood to do it, so naturally he'll also become very lightheaded the more blood he uses. He can also harden his blood into something like armor! The downside of that is that in order for it to be effective armor he'd have to be heavily bleeding. The skill also only works when he's conscious, so if he hardens or creates anything with his blood and then falls unconscious, it just turns into regular blood again.

It's very versatile as a skill but hopefully "he will pass out and ruin any and all of what he's made if he goes overboard with it" is enough to balance that out. The creation aspect seemed on theme with The Forge's style, and something something he's got blood on his hands symbolism that would make him inherently warier of using his abilities once he starts regaining memories.