^^Yup, sounds like it's been stored without pickling the membrane. I'm sure you know that bleach is a no-no around the membrane. The manual will tell you how to pull the membrane (cleaning and changing it is periodic maintenance). It should be obvious then if that's the problem.
Automatic systems use a charcoal prefilter that dechlorinates the (fresh) back-flushing water (in case you've filled your tanks from a chlorinated water supply and left the WM on 'auto'). If that filter was releasing its charcoal (say...through a hole in the filter's wrap?), it would follow the backflush pathway and come out the discharge, but not for very long.
That one's a stretch, but I can't think of what else it might be that's black and would still have the unit producing potable water.
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"Verbosity leads to unclear inarticulate things"