I work at an animal hospital we are frequently splattered with blood. Dogs don’t realize that wagging their bleeding tail or shaking their head with a bleeding ear will get messy. So we always use hydrogen peroxide, it’s handy and you can spot clean. The fresher the blood the better it will come out but it still works on older blood.
I like Amodex or Carbona (which produces stain removers depending on stain type), since it’s evidently no rush. Can’t guarantee results if you’ve treated with other stuff.
Apply hydrogen peroxide, allow it to bubble and fizz until it’s done. Rinse with cold water, reapply peroxide. Continue alternating until stain no longer bubbles when fresh peroxide is applied. Put in washing machine immediately and wash on cold. Larger stains may be permanent no matter what you try, though.
I’ve used Carbona Stain Devil and gotten pretty good results. Carbonna makes a series of different cleaners for different types of stains. Stain Devil #4 is for removing blood.
But I just checked their website and they apparently don’t sell Stain Devil #4 by itself (which they do for their other products). They only have #4 listed as part of a packet with all nine Stain Devil products.
I’d recommend checking WalMart. That’s where I used to buy it and they sold individual bottles.
eta: Or you can get it on Amazon.