Inspect the accessible rubber's condition with the appliance safely isolated and follow its cleaning instructions. A dark mark may be residue, but persistent smearing, cracking or loss of shape can indicate that the accessory needs replacement.
What the observation tells you
Do not scrub aggressively, use solvents or trim away damaged edges to improve its appearance. Cleaning chemicals and age can affect materials differently, so record any product used shortly before the change. Keep detached pieces out of the chamber.
A useful next step
Confirm the correct replacement part if deterioration is found. If the source of the marks remains unclear, send photographs rather than treating repeated staining with stronger cleaners.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.