Black labor participation rate at ~60% (2)
Black population in US at ~45M (3)
45M * (1 - 60%) = 18M
(1): http://www.pewresearch.org/fact-tank/2015/11/19/5-facts-abou...
(2): http://www.thegatewaypundit.com/2014/01/black-labor-force-pa...
(3): https://en.wikipedia.org/wiki/Race_and_ethnicity_in_the_Unit...