For hundreds of years in the US, all manner of fuckery has been used to justify the Atlantic Slave trade, and the subsequent treatment of people of African descent.
They're not actually people.
Once you've absolved yourself of any moral responsibility towards another human being, you can literally do anything to them, and it's okay.
You can torture and maim them. You can rape them, and own the children of rape. You could count them as 3/5th of a person in the governing document of your burgeoning nation.
Then you could arrest them in droves.
Once you've absolved yourself of any moral responsibility towards another human being:
You could portray them as lazy even though they were almost the entirety of your workforce; the literal backs on whom the economy was built.
You could paint their men as clowns and their women as whores, warranting your emasculation and fetishization.
You could legalize segregation, write anti-miscegenation laws, and preach 'separate but equal,'
You could poison their water, pollute their air, make them fight every day for the rights you enjoy as a consequence of being born.
You could steal and profit from their culture while demonizing them, dehumanize them when they beg you to accept the reality that their lives matter…
Or you could don blackface.
Because it's just a joke, right?