Ancient blood residue

Blood degrades. There *might* be stains on the stones, but the blood would be gone.

They'd be dark brownish splotches. If enough blood was present the entire surface could be turned dark brown on top, with only the surrounding area showing the true color. With stone, it would likely look like the rusty color is the natural hue until compared to what's around it.

You might find this interesting--blood residue found on 2000-year-old knife.

The knives are obsidian, so smoother, like glass. Coarse stone is different. (But the fact that they were able to isolate actual red blood cells is phenomenal!)

Marlys said:

You might find this interesting--blood residue found on 2000-year-old knife.

Click to expand...

Ignoring the fact this is the Daily Mail, they have pictures: The blood and hair that proves Mexicans used stone knives for human sacrifice 1,000 years before Aztecs

It took an electron microscope to identify the tissues suggesting the blood wouldn't be visible. Not that your link isn't fascinating anyway.

Red blood cells are heavy in iron, stains would likely look like rust. If one wanted a story element, one could note the rusty look of the alter differs from the surrounding stone, then note, 'iron!' for the eureka moment.

Dry or not, cave critters and bacteria would degrade the blood a lot.

Iron stains - brown splotches - is about what you would find.