I replaced the fan and installed a police spec fan clutch. Over time this is the pattern that has developed. I was thinking maybe the fan clutch, but would expect a different pattern. Car has always been maintained. No overheating issues. Any ideas? 
Are you referring to the dirt pattern on the blades? You should see the ceiling fans at my work!
What exactly are you referring to on this??
I’m guessing it’s the black wedge of dirt near the base of each blade. Migh be nothing, but since it looks like it’s on the trailing edge of the blade, it might be a bad seal in the clutch letting the silicone fluid slowly leak out.
What is the question ???
What pattern were you expecting?
Seriously, all fan blades will develop some sort of dirt pattern. Exactly what that is depends on the fan blades, the shroud, the path that the air is coming through the radiator at, the fan speed, and probably a few other factors. Air going through a radiator and then a fan blade is turbulant. And conttains dirt. How that manifests itself will vary.
If it’s operating properly, you have on problem.