Blood goo is the trail left behind by the enemies of the Blood Rooms. Stepping on it inflicts 30 damage.
Every enemy in the Blood Rooms leaves one, including the Blood Skeleton, the Blood Slime and the Blood Spider.
Why it matters
Blood goo is not affected by the Slime Ring effect that negates goo trail damage. A ring that makes Slime Pits safe to walk through will not make the Blood Rooms safe, which is part of why that floor is so much more dangerous.
Compiled from what this wiki already says on Blood Rooms, Blood Skeleton, Blood Slime, Blood Spider. No new facts were added.