Could have sworn typed a response, oh well.
If blood stains were treated with a good pre-treat enzyme based pretreat product, then laundering in a good detergent containing more of same what largely remains then is coloring. Provided blood isn't set by hot water (which in this case seems was not), before laundering or pretreating it shouldn't set.
Applying hydrogen peroxide to blood marks doesn't cause them to set in. Maybe contents of your bottle have expired? Hydrogen peroxide does lose power sitting about. If things didn't fizz when liquid hydrogen peroxide is applied to blood marks, it may have lost a bit of oooph. A fresh bottle may be in order.
You have a few options here...
Soak items again in a good detergent that contains oxygen bleach (TWB or Persil), and or use a liquid detergent along with stain booster product. Or simply soak with an "Oxi Clean" type product.
Soak in warm water using a non metal container for several hours or overnight. If marks haven't shifted, continue soaking. A fresh batch of solution will be required after about 9 or so hours.
Or, apply liquid hydrogen peroxide (from brown bottle sold at chemists) to blood marks, then let sit in well light area. Check for progress now and then, and reapply more peroxide when treated area begins to dry. Once mark is gone give things a quick good rinse.
If you have access to a washing machine that does a good very hot to boil wash, launder items again using good detergent with oxygen bleach, or bit of liquid detergent with several ounces of straight sodium percarbonate or perborate (powder oxygen bleach). Heat merely speeds up action, so what would take maybe hour or longer soaking happens faster at high temps.