Thomas Sowell (a black american economist) has some interesting videos on Youtube on the subject of slavery.
According to him, it was ubiquitous throughout history, in almost every civilisation in every part of the world, slavery was a normal thing.
It almost never had anything to do with race, across history as a whole most slaves looked like their owners. (Even though prisoners-of-war was one of the most common ways of creating new slaves.)
Thomas More's "Utopia" envisaged that there would be slaves in Utopia! Among his rules for more humane slavery were that only prisoners-of-war and serious criminals could be enslaved, and the children of slaves would not be slaves. (But apparently Utopia was a satire, so not sure if we should take this seriously.)
Sowell says that 19th century European imperialists did in fact make a unique contribution to the history of slavery - by deciding to end it. The British made the biggest contribution, once they decided slavery was wrong they sent (or threatend to send) warships to many places to end it, including in countries that weren't part of the British empire. But it wasn't just the British, the French and some other European countries also got involved in ending it. He claims this is the first time in human history that it became widely accepted that slavery was wrong. (Although my own googling found that one Chinese emperor banned it a thousand years ago, but it returned after his death.)
With regard to black American disadvantage being a legacy of slavery, I don't remember the details, but he claims that the discrepancies people blame on slavery only came into existence after the American welfare state was founded, a long time after the end of slavery. (I think he was focusing on single parenthood, which only became more prevalent among black Americans than among whites in the 1960's.) (I may not have got the details in this paragraph right, please refer to YouTube if this matters to you!)