Black mould is unpleasant, and potentially hazardous to health. It is caused by having conditions where the airborne spores can survive and thrive. It thrives in damp places. Using more hot washes and detergent which contains bleach will help prevent black mould. They also help kill bacteria that can otherwise multiply inside a washing machine. Many "colour-friendly" detergents and all liquid detergents do not have any bleach in them which is why they are "friendly" to coloured garments.
To prevent, or try to get rid of, black mould on washing machines follow the procedures for getting rid of smells and grease on this article – getting rid of bad smells from washer
Also, leave the washing machine door open after washing to let it dry out. If the washing machine is in a place where it’s exposed to warm moisture such as in a steamy kitchen then you may need to actually dry out the drum and door seal manually and leave the door closed. At the end of the day mould needs moisture and warmth to grow.
Related information: I have also written an article on – Black jelly-like substance in soap dispenser and on soap drawer which looks at what causes the slime on washing machine soap dispensers, and how to prevent and deal with it.
Removing black mould on door seal and soap dispenser
Black mould mostly grows on the door seal and in the soap dispenser. On the soap dispenser it can divert jets of water to the front and cause water to leak from the soap drawer. Check the top of the dispenser compartment where the water comes in and use an old toothbrush or something similarly suitable to remove it.
Black mould is known to release spores and there are known health hazards such as allergies and some illnesses although as it’s so damp in a washing machine it’s possible the spores don’t get released so easily – unless scrubbing them. You may be able to scrub lightly affected areas of the door seal with a scourer or old toothbrush etc. and something like Jiff. To be on the safe side you should wear goggles and a mask when dealing with black mould.
It may be worth trying a black mould remover available from supermarkets but read the instructions to make sure it is safe to use on rubber if intending to use it on the door seal. If badly affected – the only way to get rid of it – is to replace the door seal.