I hope you can take some comfort in that your grandparents are in a place where they won't feel pain or suffer. And that they're together.
If you decide to try the hydrogen peroxide method, try it on a small part of the corner first to test. It might not matter if your comforter is white but if it's not, I read that it could whiten the area? I think it depends on the material of the fabric... Here's a link I found with multiple methods of removing blood stains.
9 Ways to Remove Blood Stains - wikiHow