sandpaper and black spray paint. can't re-anodize it without stripping all the leather off.
I used to work for ALCOA selling anodized architectural aluminum. Are you sure those are scratches and not deposits? Toothpaste can remove deposits but there is really no way to remove a scratch without making a bigger mess than what you had. You could paint it, as suggested, but you'll need paint that is formulated to adhere to an anodized surface. You might try a local glass shop to see if they'll sell you a pint of black anodize touch up.