I don't know that it will come as any surprise to anyone reading here that Black people are arrested for marijuana possession a lot more than White people, despite their being virtually no gap in marijuana usage. Still this data from a new ACLU report is worth considering:

Marijuana use is roughly equal among Blacks and Whites. In 2010, 14% of Blacks and 12% of Whites reported using marijuana in the past year; in 2001, the figure was 10% of Whites and 9% of Blacks. In every year from 2001 to 2010, more Whites than Blacks between the ages of 18 and 25 reported using marijuana in the previous year. In 2010, 34% of Whites and 27% of Blacks reported having last used marijuana more than one year ago -- a constant trend over the past decade. In the same year, 59% of Blacks and 54% of Whites reported having never used marijuana. Each year over the past decade more Blacks than Whites reported that they had never used marijuana.

And now some data on actual arrest rates:

In 2010, nationwide the White arrest rate was 192 per 100,000 Whites, and the Black arrest rate was 716 per 100,000 Blacks. Racial disparities in marijuana possession arrests are widespread and  exist in every region in the country. In the Northeast and Midwest, Blacks are over four times more likely to be arrested for marijuana possession than Whites. In the South, Blacks are over three times more likely, and in the West, they are twice more likely. In over one-third of the states, Blacks are more than four times likelier to be arrested for marijuana possession than Whites. 

Racial disparities in marijuana possession arrests exist regardless of county household income levels, and are greater in middle income and more affluent counties. In the counties with the 15 highest median household incomes (between $85K-$115K), Blacks are two to eight times more likely to be arrested for marijuana possession than Whites. In the 15 counties in the middle of the household income range (between $45K-$46K), Blacks are over three times more likely to be arrested for marijuana possession than Whites. In the poorest 15 counties (median household incomes between $22K-$30K), Blacks are generally 1.5 to five times more likely to be arrested...

The overall Black population has almost nothing to do with this disparity:

For example, in Lycoming and Lawrence, PA, and in Kenton County, KY, Blacks make up less than 5% of the population, but are between 10 and 11 times more likely than Whites to be arrested. In Hennepin County, MN (includes Minneapolis), and Champaign and Jackson Counties, IL, Blacks are 12%, 13%, and 15% of the population, respectively, but are 9 times more likely to be arrested than Whites. 

In Brooklyn, NY, and St. Louis City, MO, Blacks comprise 37% and 50% of the residents, respectively, and are 12 and 18 times more likely to be arrested than Whites. In Chambers, AL, and St. Landry, LA, Blacks account for more than twice as many marijuana arrests (90% and 89%, respectively) than they do of the overall population (39% and 42%, respectively). In Morgan and Pike Counties, AL, Blacks make up just over 12% and 37% of the population, respectively, but account for 100% of the marijuana possession arrests.

Read it at The Atlantic.