I have LOTS of experience with this, not only on quilts but on clothes.
Now, we're talking about a "spot" not a glob. I have often had nosebleeds that spot the front of my shirt. If you spit on it (and I've even sort of sucked in and out on the fabric) it disappears almost entirely. Of course you would want to wash it in mild soap and rinse with clear water soon after.
As another member said, this only works on your own blood, not on anyone else's.