Whether it be a starter or alternator or fuel pump that failed and you take it apart the commutator plane would most likely be shiny where the brushes make contact unless it was corroded with some substance.Or sometimes the brushes freeze causing a jumping spark that causes discoloration. Judging from the photo it looks like it was sitting a long time with all that black stuff in the photo.
Yes that would be considered the commutator plane where the black stuff is.