Black yet firm root base; mold or infection?

[itzi]

I was re-potting a pretty healthy looking phal into some new rePotMe medium, and I noticed one root has a black/gray base. It looks different from the rest of the brown spots; its still plump and firm, and actually rather a dark gray color. The rest of the root is yellow and I think there's even new growth farther down.

Could this be an infection? Or a type of mold? I dont think theres anything spreading to the rest of the plant. Theres no black spots on the stem or leaves. It was potted very tightly in sphagnum moss from the supermarket so I'm assuming its related to that.

Thanks!