Not all chlorine bleach sold for laundry/cleaning purposes will kill mold. Many aren't even effective disinfectants any longer either. This is because they have been weakened and thus no longer contain as much "chlorine" as they once did. In the USA it is important to look for the EPA register number on the container. That and read the label to see if the product is designed for killing mold.
Thing about mold is it is a living organism and actually grows roots into surfaces. This why it is often so difficult to remove it totally. Substances may take off and or kill the top layer, but the roots will send up replacements.
Quite honestly the best and safest thing to do when a washing machine's boot is in such a state is to chuck it and replace with new. As another poster stated if what you can see looks that bad then God only knows what is growing underneath.
When my vintage Miele arrived was glad to have ordered a new boot to be on hand for the installation. Not only was the current one mouldy on the inside (where one can see), but when the repairman got it off you should have seen what was growing underneath. It was one gross slime fest of mould and rot.
Thing is once you've got mould growing in a washing machine it will eventually colonise a good part of the machine as the spores spread. This plus not to mention whatever is being laundered may become contaminated as well. Oh and leaving the door open and or using the machine can cause spores to spread around the area near the machine and probably the room/house as well.
If you do not wish to change the boot then shop around for a certified mildewcide or try white vinegar, peroxide or some of the others mentioned here:
http://www.wikihow.com/Remove-Mold-and-Mildew
Of course the best method is to start the way you mean to finish. Door of the washing machine must be kept ajar to promote air circulation. Next a check of laundering habits is in order. If the household is using mainly cold or cool water washes with liquid detergents it is imperative that on a regular basis a hot to boil wash is done using a powder detergent containing (oxygen) bleach.
Many liquid and gel detergents contain good amounts of "soap". This residue plus bodily oils and other fats found on laundry can and often do leave a nasty mess that germs and mold/mildew love to feed upon.
Powder detergents contain a good amount of alkaline builders (washing soda) that will break down grease and oils helping to keep the washer clean.
If powder laundry detergents are not on then a routine maintenance wash should be done using boil wash temps and soda crystals. That or various washing machine "cleaner" products. The latter should not just be designed to descale, but remove build-up as well.