So?.....
....we're talking about relatively large dark particulate matter on the lee
of the hill....
Aeolian driven origin?
I don't think so. Martian winds wouldn't appear to budge the heavy dark stuff.
Deposited by ancient explosive eruptions of gas/vapor from now buried vents in the hill?
Maybe.
Hydrological depositing?
I would side with Nick on this one, and say NO.