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 on cotton sheets, 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 proceeding. This will likely cause more harm than good with delicate items. As NeilM138 stated below, if the item is not bleach safe, do not attempt. 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. Let sit for 5 minutes or so. If the stain hasn't been set by the dryer yet, you can now wash/dry in your usual laundry cycle and it should disappear. If the stain has been set by the dryer already, proceed to Step 3.

Step 3: Add Heat

Once it's had a chance to soak for a few minutes, take your hot iron (it needs to be on the highest setting) and go over the spot until the stain disappears. You may notice a ring around where the peroxide was - this should go away once you've put it through the wash again.