While I don't personally worry about stains like that, this will help avoid them in the future, IME:
Soak in a bucket of hot tap water with cheap dishwasher detergent until all the blood is out of the material and any meat bits are basically digested. Dishwasher detergent has enzymes that break down proteins and does a great job at completely breaking down the blood and removing it from the material.
Usually mine are clean enough after this I just hang them to dry after rinsing. If not clean enough for you at that point, run them through the wash. Don't use the dryer, as stated previously, that will set any stains.