From 'Spotless 2' by Shannon Lush. (wouldn't be without it) 'Blood is a protein stain. Dampen a cake of bathroom soap in cold water and scribble over the stain as though using a big crayon. Scrub in every direction with a toothbrush. Wipe with a damp cloth, place paper towels on top and stand on to remove moisture. If you have applied any other product to the blood, neutralise it first by wiping with a cloth wrung out in white vinegar. Put a couple of drops of glycerine on a toothbrush, scrub over stain and leave for 20 min, then use the soap and water technique as described'. I guess you could use a nail brush or something for a biggish area. Don't saturate through to the carpet backing as it could discolour the carpet. I have gray wool carpet and any cleaning always leaves a darker area which fades in time even with a very light spot clean.