Black sludge in washing machine help

When a wash has finished I always put some folded kitchen roll inside the rubber seal at the base to soak up excess water. That helps a bit

 

Have you tried a bicarb paste John?

 

Don't get similar marks but if the drum starts to smell a bit stale we do a high temp wash with a bit of household / toilet bleach added in and let it go though all the rinse cycles, seems to do the job.

If yours is gunging up so badly, might be worth checking you drain pump filter / assembly, it might be full of gunk etc .

Agree with @Bluejayway , dry off the bottom of the inside of the door seal where if you open up the seals rubber folds a bit you will see a little drain hole which should be clear and dry.
After we have finished the washes ,we leave to door and dispenser tray fully open , usually overnight to ensure it dries out fully.

 

Vanish oxi action powder in a quick wash should sort it out. Remove the dispenser tray if there is one and wash it in the same stuff. It should also clean the duvet cover too.