This slip ring is perfectly serviceable from a wear and coloration standpoint. I would not turn it down as that gains nothing. However I would deduct the ledge wear from the min acceptable brush length. The brush path is supposed to look slightly blackened, this is carbon brush transfer that provides lubrication.

There is a whole black art when it comes to electrical device brushes.

Also note the difference in wear between the two. I'd expect the slip ring with the most amount of wear to have the least worn brush. This is due to the direction of current flow. One brush / slip ring is current in and the other current out.