I think you’re seeing the mucous membrane lining on the deep sinus bones. Can be hard to get out. Sometimes you can scrape or rub it off with a needle tool or something, or flush it with a strong stream of water. There’s often something left in the sinuses, and the brain cavity. There will also be stuff left in all those little openings where nerves/vessels go through. Not the end of the world. It will bleach up mostly, but might smell for a bit. It will eventually dissipate. If the bones are warping or separating, stop with the heat. It’s just going to get worse.
After you bleach it and it’s dry, spraying with a satin or matte clear spray can help seal it and keep the smell down.