The door had been unlocked as an invitation. John might not have wanted much to do with magic when he'd gotten here, but even he wasn't ignorant enough to avoid the basics out of stubbornness. His wards were hidden but drawn -- no one walked into his place without him knowing about it.
Didn't stop him swearing one more time, sucking blood from his finger before turning to offer Natasha something of a sardonic smile, all crooked and charming all in one go. "No," he disagreed. "This is 'bout right. The sharp point ends up in some part of me, generally." He didn't miss much of a beat putting the knife down and leaning up against his counter top. "Wotcher, then?"