I've tried many blue filters for blood tracking game and came to the conclusion that they're all lousy. Blood only looks black under blue light and so appears as every other black speck on the ground. This is because blood absorbs blue and UV light and does not fluoresce, except when it's been spun and separated into plasma, which doesn't happen in the woods.
Incandescent to red light does tend to reflect well off fresh blood with the key word being
fresh. After a couple of hours it becomes more difficult to see and the ambient temperature has a big effect. The warmer it is the quicker it turns dark. The colder it is, the longer is stays a nice bright red. In short there is no magic wavelength to illuminate dried blood, contrary to some of the more outrageous product claims that I've seen.
Along with incandescent lights I also use gas and propane lanterns since that wavelength seems to work pretty well on red blood. The problem is that the light from the mantle is omnidirectional and so it has to be very close to the work area to be helpful, otherwise it's two diffused. I usually hang a couple of lanterns in trees and then get down on my hands and knees with the flashlight if the trail is really difficult to follow....which we'll assume is since any light will allow you to track a gusher.
I find that the biggest benefit comes from having light sources from two or more angles because this system helps to eliminate the harsh shadows. Having lanterns hung in the area, then using a headlamp as well as a hand held light helps with this. The other technique that I'll use is sweeping the beam back and forth in front of me because the fresh blood tends to flash back a bit because it's so bright red compared to anything else on the ground. For some reason the eye picks up on a quick flash of red better than if I stare at one area with a still beam.
The last thing I wanted to say is that it sometimes just takes practice. When I first started tracking difficult blood trails I absolutely sucked at it. It took me several times of trying to locate animals in the hunting party before my eyes and brain synchronized and became used to looking for the stuff at night. Bright day light was never a problem of course, it was just those times when the animals are taken right before sundown.
Deer, elk, antelope and others can cover almost a 1/4 mile in little over 20 seconds if they're booking. I always leave the area quietly and bring my buddies back if the animal runs beyond visual range. Fortunately most of my archery animals have fallen within 50 yards but there is always the exception no matter how much care and effort goes into a "perfect" shot. My last archery bull elk took 3 people three hours to find even though it only when 250 yards. Even though the arrow zipped straight through, the animal's thick main acted like a sponge and soaked everything up from both sides. I was sure relieved went my one buddy who was scouting up ahead yelled, "I got him!!!"
