Where the water muddies.....is when the blood has a chance ot sit for a while. It will start to look the same, then. The contrast in color is most important when you're checking your arrow at the POI. If you see the bright stuff (because most people aren't going to wait too long to at least check their arrow -- refer to Rob's post for timing to do this)......you're more likely to know you can get on the trail, sooner.

If you can't tell.....or you don't see the bright stuff......

Go home. Come back. If he's dead....he'll still be dead becasue of your wait.