From the New York Times and The American Lawyer - the more you think about it the more disturbing these numbers are:

"Black lawyers accounted for 3 percent of lawyers at big firms last year, a percentage that has declined in each of the last five years. And the proportion of black partners at such law firms remained stagnant at 1.9 percent during the same period, according to the 2013 diversity scorecard published in the June issue of The American Lawyer." (link)

The black population of the United States is currently 13%. 

As previously discussed (here and here), subconscious biases likely play at least a part in these dynamics: 

"Blacks lag behind in the top tiers of the law, according to the American Lawyer, because unconscious racial bias can influence the types of assignments and the relationships between and among employees. That can hinder black lawyers from advancing to the highest slots, it said."  (link)

As explained in The American Lawyer:

"What still is lacking, many black lawyers and diversity directors say, is a broad commitment by individual white partners to ensuring the success of minority lawyers, and particularly black lawyers. Recent research has painted an alarming picture of the continuing presence of unconscious racial bias at firms. The research confirms what a lot of black lawyers have known all along: It's not enough to recruit more black associates if you don't deal with pervasive bias." (link)

As explained by a black female ninth-year associate at a midsize firm:

"'You are deemed worthy of receiving the keys when you are liked, and you are usually liked by people who can relate to you or perceive you as similar to themselves,' a black female ninth-year associate at a midsize firm says, asking not to be identified because she is up for partnership. Black lawyers, she adds, 'would more often than not say that they were not able to bring their whole selves to work and therefore grew tired of the ruse and moved on, or they brought their whole selves to work and found themselves ostracized and alienated.'" (link)

The American Lawyer article points to a study similar to one previously covered here (regarding email responses by professors at top colleges):

"In late April, law firms were roiled by a study that shows in the starkest terms yet how implicit bias remains pervasive. The study, by Nextions, a law firm diversity consultant and leadership coaching firm, found that supervising lawyers were more likely to perceive African-American lawyers as having subpar writing skills.

In its study, Nextions inserted 22 errors, including minor spelling or grammar errors, factual errors and analysis errors, into a research memo written by a hypothetical third-year litigation associate. The memo was then sent to 60 partners who had agreed to participate in a writing analysis study. Half got a memo identifying the author as African-American; the other half, a memo noting that the associate was white. The hypothetical black associate got a significantly lower score on average than the hypothetical white one. Partners, regardless of their race or gender, had more positive things to say about the work of the white associate, and found fewer mistakes on average in the paper."  (link)

Of course, that is not the only dynamic at play:

"Interviews with two dozen black lawyers, in-house counsel, diversity experts and academics, plus our exclusive law firm surveys, suggest a variety of causes. Most agreed that pressures within law firms that began during the recession have made partnership both a more difficult and less attractive proposition for black lawyers. Meanwhile, the pipeline has narrowed. As firms keep associate classes smaller, fewer black lawyers are moving into firms; the black law graduates who are tapped by elite firms continue to be a small group of high-ranking students from first- or second-tier law schools. Finally, a mid-2000s push by corporations to compel their outside counsel to diversify has receded, displaced by concerns over law firm pricing."  (link)

The American Lawyer article highlights some of the efforts some law firms are taking in response to these issues, including revisiting hiring and evaluation systems:

"In the meantime, firms such as Schiff Hardin, Littler Mendelson and Reed Smith have begun taking steps to address both unconscious bias and structural impediments to black lawyer advancement. These steps echo in practical ways those recommended by the American Bar Association's Presidential Initiative Commission on Diversity in 2010 in its report summarizing its findings after a year of hearings on the issues." (link)

The full American Lawyer article is available here and is a very thorough and thoughtful piece.