The "white man bad" was in reference to some widespread attitudes that white people have some form of collective guilt for slavery. At the same time, people play mental gymnastics and use some passage of years to excuse the people who actually were directly responsible for it.
It's very peculiar how it's OK to blame some greater group, purely because of skin color, but blaming those directly responsible is deemed unacceptable.
Someone please try to explain how the same argument would work with Germans vs Nazis. C'mon, you leftists love anything with the word nazi in it ...