You can either clean the aluminum, get some Aluminum Primer from a boat store, (Zinc Chromate, $9.00, is not
sold in auto parts stores) mask off the panel, prime and
paint the frame.
I am sure there will be a day, when you will wish that the
solar panel was working ! With some extra wire, it could help charge your vehicle
battery. If you were to sell the trailer, that
solar panel is a great selling point on a
U-Haul.
The other option is to remove the panel, fill the holes with Bondo Spot (in a tube from Walmart auto dept, under $3), sand when hardened, prime, and
paint. Then sell the
solar panel.
I am sure that there are other ways to address this blackening problem, but these are my first two cents. Penny