Oof sorry, this is my engineer coming out and being picky about this. If the blood flood (lol) is extremely dense, wouldn't that mean that any human caught in it would be more likely to float due to differences in density? Even if their own blood is replaced with that ultra-dense blood, their other constituents (fat, bone, any air still in their lungs, etc.) will almost certainly be less dense, giving them buoyancy. (If you imagine that a human is ~70% water but can still float in freshwater areas, same thing should apply in this blood). I think that if the flood really is that dense, if it's a fast-moving liquid like a surge, any person would much more likely be outright crushed by the aggressive flow than drown. Just my two cents on that.
But anyway, other than my physics brain going wild, this is a really fun article!! Very cool new phenomenon to have in the Backrooms.