Would it smell?
What, if anything, could grow there?
How much blood can soil absorb, and long does it take to dry?
How many victims at a time and how often the rites are performed are related to what I find out about the above. I don't want a situation where the soil would be constantly soupy nasty mud but I'd like the temple to be creepy.
I tried Googling variations on blood absorption, but that got me things entering the blood, not the blood entering anything else. I also checked sites and books about forensic blood analysis and some of the experiments done on the Body Farm, but none of that was helpful in determining what the ground would look like or the long-term effects of repeated soaking. Finally I checked the library for information on Aztec sacrificial rites since they were the bloodiest I could think of, but nothing on what happened with the blood.