Our 2006 Altima SE-R is black and guess who maintains it's appearance? Yep me.
Best thing you can do is
hand wash it only (no automatic car washes unless they are the brushless kind).
Use a GOOD car wash soap, not the $2.99 a gallon stuff and a microfiber mitt or towel.
And don't skimp on soap when you fill up the bucket, that's what lets the mitt or towel glide over the paint and not drag.
From there all you can really do is keep it waxed and use compounds/polishes to remove anything you don't like in the paint when you find it.
It takes work, but black is a great color when it's clean: