Personally, I find my black sand super easy to maintain and it doesn't look all that dirty to me, unless I slack on my cleaning.
However, if you don't like the way the sand looks, I'd just replace it rather than mixing two styles and colors of substrate. Personally, I think it would look terrible, but that may just be me.
Also, adding a layer of gravel over your sand will likely create problems. When you gravel vac gravel, you have to do "deep" vacs from time to time. This won't work so well with sand, and you'll end up siphoning a good amount of the sand out each time you vac. With sand you should hover over the surface and all the gunk comes up, but most of the sand falls back down. I think this would be an invitation to trouble.
\Dan