Pipe stem material is made black with dye. Dye fades over time, depending on the chemistry of the stem material, exposure to UV, etc.
You can restore that deep black (and remove a minimal amount of material) by immersing the stem in Fiebing's leather dye. I clean the stem with alcohol to remove any oils, then run a pipe cleaner through it and suspend it in a bottle of dye for at least 24 hours. The longer the better. Remove and let dry for a couple of days (depending on air temp, flow and humidity), buff and BINGO.