Black build up

[InTheShade]

Did you ever see it not like this? It looks like maybe that is from the way the mod is made, looks like raw unfinished metal, perhaps from casting, they just didn't take the time to sand and polish the inside of the mod

That's exactly what I was thinking, the build-up is not on the threads and there is a nice clean line between the coating and the threading. This made me think it was made like that.

 

[Singaw]

Looks like an oxide aka rust. Carbon should wash right out and shouldn't be in there anyway. Like another poster said it's possibly unfinished. Mark it up to poor quality control I guess. A flex hone would clean that up in a jiffy but probably cost more than the mod.