Here's more info. i'm in southern california where the chinese elms have
budded out since early February. I keep the plant outdoors. the black stuff
does not come off or crumble. I water the elm when the top 1/2 gets dry. NOTE:
this is the only elm i have that is having this problem to an annoying degree.
it seems very healthy. it's not in Bonsai granular bonsai soil and it seems to
be in potting soil (good soil tough). I have 4 Chinese Elms. 2 In Bonsai soil
(1 repotted myself) and 2 which I bought recently in regular potting soil. One
of the potting soil elm is not afflicted with the black spots but the other one
is.

As per a suggestion, I went to Walmart and bought a bottle of Orthenex and
watered it with that today. Hopefully that helps.