I always wondered; why are the bodily fluids, etc. that accumulate (and usually dry out) in the corners of a dogs (and cats too, I think) eyes black?; but in humans, it is not?
This is common to many dogs I've been with.
This link http://m.petmd.com/dog/general-health/5-types-dog-eye-discharge-and-what-they-mean talks about several different kinds.
I believe the one you're talking about is the dark brown/red crust (#3)
Light colored dogs often develop a reddish brown discoloration to the fur below the inner corner of their eyes. This occurs because tears contain a pigment called porphyrin that turns this reddish brown color upon prolonged exposure to air
Humans tend to keep themselves cleaner than dogs do. The difference in color is dirt buildup. Ask any girl who doesn't wipe off her mascara before she goes to bed what color her eye boogers are in the morning - black from the buildup!
Of course dogs don't wear makeup, but they DO dig in the dirt, do their business outdoors, go for walks, stick their noses and faces into your garden and anything else that fits their fancy! All this is done without a shower every morning or evening. If you started a daily routine of wiping your dog's eyes with a "pet eye wipe", the discoloration will become significantly reduced.