There not only is BLACK colored truss, but many other colors as well!
Alumimun truss is more comon because of two main issues, Maintenance and Look. As others have already said, if you scratch standard aluminum truss, it still looks good. In fact, from 10-20 feet away, even really banged up/scraped/nasty aluminum truss looks great to the human eye!
Black or other colored truss, no matter how the finish is apllied will not look as good after it is scraped and dinged. On the road, it is very hard to keep small dings and nicks off the truss. Trusses get slid into the truss or stacked ontop of each other or on road cases. Nicks and dings and scratches are a given on a road show.
The other main reason you see aluminum truss more is the Look of it. While you may prefer the look of black truss, many designers and clients prefer the "industrial" look of aluminum truss and aluminum cans.
On concerts, you will probably see either aluminum or black truss and cans, with aluminum being more popular. In theatre type gigs, you are more apt to see black truss and cans (and lekos, fresnels, etc).
At trade shows, you will see a lot of truss in many different colors. Trade show exibiters like to color coordinate their booth displays to their company logo colors, and the truss is no different. Recently at a trade show in Vegas, I remember seeing truss in RED, GREEN, BLUE, YELLOW, and POLISHED ALUMINUM in different booths. display truss is usually smaller sized and lighter weight than concert type truss, so its easier to handle carefully so as not to scratch or damage the color. Many display truss structures actually pack up into trunks or shipping crates or on pallets for delivery, so the pieces dont get damaged. Also, display trusses may be used only a few times per year, so the colored truss will stay looking good longer than concert type truss whick may get loaded/unloaded in trucks on a daily basis.
Craig