I'll try to do my best Sherlock Holmes, here . . . First off, whatever is causing the smell isn't necessarily the same thing that's causing the black marks. Seems like there's a good chance that it's the same thing, but it's not a necessary conclusion. So let's take them one at a time, starting with the smell (though the same analysis will apply to the black marks#.
Whatever is causing the smell has to be #a# something that's part of the washer; or #b# something that's been introduced to the washer, which could be either #i# something that's growing in the washer like mould or mildew, or #ii# something that is introduced to the washer each time your mom uses it; or #c# a combination of #a# and #b#.
If you've eliminated oil or grease used to lubricate the washer, that would seem to eliminate everything that fits into category #a#, at least that I can think of. So that leaves #b# or #c#. I can't think of what combination of internal and external causes would create the smell you're describing, and, frankly, this category seems like a long shot to begin with, so that leaves us with category #b#. Either you've got some kind of mysterious mould or mildew that you're not familiar with, or your mom is putting something in the washer that's causing the smell. My best guesses as to what the later might be are that something is wrong with the detergent she's using, or she's washing something that makes everything else smell #e.g., oily/greasy clothing like a car mechanic might have), or there's something in the pipes that provide water to the washer.
Hopefully this gives you some food for thought.
