Quite often caused by a reaction between the soap powder & liquid softener which causes a build up of a blackish scum between the inner & outer bowls. To clean it properly, the inner steel bowl needs to be removed & scrubbed clean. while it is out the outer plastic bowl can be scrubbed as well. If you don't have anyone capable of this, you could try running a hot wash with a descaler solution in the water (No clothes) You can buy it from an appliance store. It is also used for dishwashers. But the best way is to remove the bowl.
Lincoln Appliance NZ, December 2008
