Warning: Do NOT use Warm or Hot water to clean the blood! It'll set in and you'll never get it out.

Try cold water (colder the better, put ice in it) mixed with oxy-clean. No bleaching effect, minimal scrubbing - don't soak the area or it'll just seep through the carpet and damage the floor beneath. Wet it down until it's nice and damp, then hit it with a scrub brush, then rewet and repeat until the stain's up... then be sure to sop up whatever moisture you can with towels.

Good luck to you!