The story
Uncertain. Perhaps from blackedy (“characteristically black”) + black, with reduplication serving to intensify or stress the focus on being black (i.e. African-American); blackedy being equivalent to blacked + -y; or alternatively a corruption of blackedly black. Compare blacked up. For the spelling with -ity, see -ety, -ity.
Etymology from Wiktionary, CC BY-SA 4.0.