How do I clean blood stains out of carpet?

Here's a couple of ideas that may help. One uses dishwashing liquid, the other uses ammonia.

https://spotremoval.coit.com/how-to-get-blood-out-of-carpet

Try using Peroxide. Check a spot where it doesn't matter if it should lighten your carpet. It takes blood out of lots of other material. Good luck.

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I would try blotting it with hydrogen peroxide

Hydrogen peroxide will do the job.

Grab your bottle of hydrogen peroxide! Just apply a small amount of hydrogen peroxide directly to the stain and watch as the red blood stain disappears. In the case of old or stubborn stains, reapply as needed. After the stain is removed, rinse the area with cold water to remove any peroxide that may be left behind

Funny thing is that a person saliva is the best thing to get blood out but if it is a lot of blood use cold water over and over. Once you think you have gotten as much out as possible then use Greased Lighting. https://amzn.to/2YlUzz8