According to Variety, it was first posted on Twitter by a user named chefboyohdear, and showed Fallon, as Rock, appearing on a talk show.
Fallon tweeted that it was a terrible decision to impersonate Rock, also a former Saturday Night Live cast member.
I am very sorry for making this unquestionably offensive decision and thank all of you for holding me accountable, the late-night star wrote.
The history of minstrel shows and movies where white performers donned blackface to mock blacks has made its use considered offensive today.
Sarah Silverman used blackface for a sketch on her Comedy Central show in 2007, a decision she later said she was horrified by.