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Could it be from oxidation inside the mast - it seems to build up as a fine black powder.
Strictly speaking, machinists call that smutt, and it is black. More like micron sized particles that retain a black color even after oxidizing. Something to due with the refraction of light than the true color. Chemically passivated aluminum surfaces often turn black for the same reason (micron sized imperfections that trap light). Some engine coolants cause this black staining of aluminum.

Just a small amount of borax in the rinse water will keep the mildew from returning if protected from rain. A few tablespoons per gallon.
 
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