Strange doesn't say anything. Instead, he takes the paper from Rita and signs his name on it. There we go, the contract is made. And, now that he's signed it, he starts to feel an odd little compulsion at the back of his mind, something which he can only assume is the contract taking shape. He delays for another second before he finds himself answering, without even thinking,

"It is," he admits in a matter of fact tone. "The quill magically writes using the blood of whoever's holding it."

Something which doesn't seem to rattle Strange all that much. Okay, so it uses blood. Plenty of magic does the same! That's his justification and he's sticking to it! "Does that bother you?"