I'll admit up front, I disagree with President Obama's policies and I rarely read the Washington Post. I only found your article through a link on the Drudge Report. I have avoided sharing this picture when it is sent to me, as I find it would only cheapen my valid criticism of the President's policies.
 
At 28 years old, I cannot fathom any of my peers reaching your conclusion about this poster, be they red, yellow, black, or white. Although my interpretation of the poster is as much speculation as yours, it is very likely the creator is nearer my age than yours. My immediate impression is that the poster maker simply took a recent example of a widely recognizable villain in order to suggest that the President's agenda is not only radical, but that he, himself, is dangerous.
 
Your view of the poster as racial commentary not only appears contrived, but your further connection of the Joker with urban black crime is ignorant. The Joker is the creation of 1930's America, when Irish and Italian gangs were the criminal threat to our cities and white, male bank robbers captured the public imagination. The cultural association of black people with urban decay is, as you say, a product of post-integration 1970's America, two generations after the Joker was first imagined.
 
Further, your suggestion that the "Dark Knight" Joker is a stand-in for urban blacks who carry the "virus of violence" is ridiculous. First, although residents of the East (New York, Philadelphia, etc.) may wrongly view black people as the face of urban violence, residents of California's great cities, for example, are just as likely to see asian or hispanic people in their mind's eye. Second, the dichotomy between Jack Nicholson's portrayal and Heath Ledger's is a clear result of generation gap, with the former reflecting on the 1960's sitcom villain, and the latter purposefully presenting a realistic contrast to the former.