Here's a neat trick to get blood stains out of white sheets even after they've been set by the dryer. **EDIT** The following instruction is what worked for me, but please read through the comments before doing this. Some users found that using higher-strength peroxide resulted in tearing or fraying of the fabric after washing, and others have modified this instruction with different results. There is a lot of great feedback from those who shared their experience using this method, so please review the comment section before attempting. Thank you to all who have contributed.
Step 1: Materials
All you need is some hydrogen peroxide, a clean rag, a spray bottle with clean water, and a hot iron.
Step 2: Soak
Pour peroxide on the stain and give it a good scrub with the clean rag. Feel free to use as much peroxide as you're comfortable with - you want to really give the stain a good soak, followed by a good scrub. Give it one last splash, and let sit for 5 minutes or so.
Step 3: Add Heat
Once it's had a chance to soak for a few minutes, take your hot iron and go over the spot until the stain disappears. You may notice a ring around where the peroxide was - just wet it with the spray bottle and go over it with the iron again. Voila!