Actually if your tracking a fresh kill the blood should still be wet and bright. As the blood dries, it gets darker and harder to see. The peroxide just helps locate the blood you can't see by making it bright again. Of course the foam will be slightly larger than the original spot of blood due to expansion. It's not foolproof but helps when you have totally lost the trail but have the last spot marked. You just spray wide strokes and hope the foaming shows up under your lights.