It's not blood. It's Mioglobin.
What Burger King is doing, is that, it leaves meat at 50/60°F for a week. Then, the meat will turn brown because it's releasing Mioglobin, which is the red liquid that sometimes you see in bags with meat.
After this, they collect this Mioglobin, soakes the buns in it to give it a red color, and then use the week old meat for the patties. Fly larvae, maggots and spiders are rich in protein and adds volume to the meat.
So... basically yes, literally a spider-verse meat.