Or, maybe the composite material itself is black. I have a composite material made from graphite. That piece is black.
I wondered that, too, a lot of Graphite stuff is black, but look at the outside surface: GREEN, most that I have seen in green colored, the graphite I have seen in spars is greyer and only is a layer or so, not through the materials in all places.
I say the black is a heat damage from the incident of shearing, failure itself. Too many sharp edges.
For the record, fatigue would NOT MAKE EDGES DISAPPEAR, but the sharpness would NOT remain over time, it would on the outer edges of th material, you can see a fraying of the material like a wisping of thematerial, but not inside, that would be pressed and opened many times, it would be reduced in sharpness. I see too much sharpness.