"Yes, I see what you mean." Cas answers grimly, looking over the wound, now certain he knows what it came from. The shape and nature is plenty familiar to the First Blade, it seems pretty conclusive, but the scent still baffles him. "You have a good eye for mortal wounds. Is this something you do often?"

Cas doesn't get the impression that Blake is a hunter, or familiar enough with the supernatural to link sulfur to demons. He'd rather rule out any other possibilities before following the assumption of a demonic presence here. Strange as it is to find hints of demons, it doesn't link this incident to Dean, unless the mark has somehow evolved to summon them. The wound is much more telling, ease to match to the weapon, but this new element needs to be looked into.

Sure, he'll share his thoughts, on the sulfur, at least. The nature of the wound he'll feign ignorance for, but the scent is enough of a lead (that won't link back to dean) to occupy them.

"Do you know of any reason sulfur might be present? A volcano, or mine, or factory nearby?"