A friend just e-mailed me this "guidance" on visiting Washington, D.C., from a Maine Tea Party website, which basically goes out of its way to impose on Tea Party activists the necessity of not accidentally visiting any of D.C.'s mostly black neighborhoods:
If you are on the subway stay on the Red line between Union Station and Shady Grove, Maryland. If you are on the Blue or Orange line do not go past Eastern Market (Capitol Hill) toward the Potomac Avenue stop and beyond; stay in NW DC and points in Virginia. Do not use the Green line or the Yellow line. These rules are even more important at night. There is of course nothing wrong with many other areas; but you don't know where you are, so you should not explore them.
If on foot or in a cab or bus, stay in Bethesda, Arlington (preferably north Arlington), Crystal City, Falls Church, Annandale, or Alexandria, or in DC only in northwest DC west (i.e. larger street numbers) of 14th or 16th streets, or if on Capitol Hill only in SE Capitol Hill (zip 20003) between 1st and 8th Streets, not farther out than 8th (e.g. 9th, 10th etc). (Or stay on the Mall and at the various monuments.) Again there are many other lovely places, from the Catholic University of America to Silver Spring, Maryland. But you don't know where you are so you cannot go, especially at night, unless you take me with you.
I suppose if you exist in a media atmosphere in which black people spend their time developing plans for racial "payback," then believing its really dangerous to enter a mostly black neighborhood is an entirely rational response. But generally speaking, I'd say a white person walking into a rough neighborhood is probably the safest person there. Most likely, someone's going to care if something happens to you.
Of course if you do plan on venturing into the urban wasteland, you could easily ask TAP writers Tim Fernholz or Jamelle Bouie for advice, since both of them live on the Green Line and have thus far managed to avoid being shot to death. That could be because tales of violent crime in D.C. are overblown and upper northwest D.C. is rapidly gentrifying or because they're deadly urban guerrillas who would be quick to leave you in a coffin, for slick talkin'.
UPDATE: DCist has a helpful Google Map.