Why are there black marks on the magnets after washing?

This may happen due to the following:

There is a high iron/metal content in the water, or there is a high metal content in the clothing itself. This is typical of people that work in metal fabrication, etc. 

Either way, the MLS units are pulling the metal out of the water and garments during the wash cycle, and they are collecting on the outside of the units. The way to remedy this is to scrub each MLS unit in the sink with a scrub sponge every few days to keep them clean, and to remove the metal debris buildup.

This does require a little bit more effort to maintain the MLS units, but the positive is that you are able to remove the metal from your clothing and water so it doesn't ultimately damage your machine and collect in your clothing.