That doesn't look bad enough to significantly affect performance. Most old motors get to looking that way. It's much more important where it attaches to the terminal screws. Make sure it looks clean under there.
Hard to say from the pictures. To me, the most important thing is the condition of the commutator. Brushes are easy to replace, and bearings are not too hard either, but if the comm is bad, it's a lot of work.
The copper wire coming from the brush does not rust. Rust is iron oxide. Corroded copper is green or blue-green in color. So if the substance on the copper pigtail is rust, it came from elsewhere and is of no consequence or could be cleaned off.
If on the other hand, what we're looking at is discoloration of the copper different from the adjacent brushes, then it could be due to excessive heat. Likely cause is unequal current sharing. Current is split between 4 brushes; adjacent neighbor and 2 at 180° opposite on the comm. Check and clean the connections, make sure the bush slides freely in the holder, all spring forces are equal and brush face seating is equal amongst the 4. Also check the pigtail connection to the carbon/graphite block, called the tamp. If loose, replace brush. It actually does not look like a severe problem from here. If worried, you could trade positions with one of the other brushes in the motor and see if the problem follows, stays or disappears.