One day my little friend Sydney caught a bird and left its bleeding carcass at the back steps. (She loves me, oh yes she does.) It lay there for several hours before I found it, and by that time the blood had soaked into the concrete.
I washed the spot with a bleach-based bathroom cleaner, and still the bloodstain remains. You can see how the cleaner killed the lichen and bleached the area immediately around the stain.
(The lichen is a welcome guest, by the way, and grows on the vertical sides of the steps and in broad patches on the patio itself. It will eventually eat through the concrete, but I enjoy watching it change through the seasons. The lichen gets a little furrier during the winter, flakier in the summer.)
Why on earth did I file this under Writing? Research, of course. You never know when you might need to know what a pool of blood — and that bird had an amazing amount of blood for a critter so small — does to concrete.