what if john Browns raid on harpers ferry had succeded in starting a slave revolt throughout the southern states
1) The rebellion is crushed. All of the Southern States had trained militias, which would have cooperated with the U.S. Army to crush the revolt. And, except in South Carolina and Mississippi, whites outnumbered blacks by a significant margin in all the Slave States. Very few of the slave rebels will be armed with anything besides clubs and whatever farm implements they can steal. Brown won't have time to drill, train or discipline them. Against an organized military force, they are dog meat.
2) Lots of innocent white women and children die, killed by Brown's rabble of revenge-crazed slaves, which he won't have time to properly train or discipline. So they will basically, as happened in previous slave revolts, kill any white person they find indiscriminately.
3) Public opinion across the nation, North and South, is horrified at what Brown has unleashed. Most people conclude that blacks can't be trusted with freedom...surely if they are set free, they'll go on the rampage and murder every white they can get hold of! Abolitionists across the North are lynched or driven into exile.
4) If you thought slavery was bad before, you ain't seen nothing yet. Regulation and control of the surviving slave population will get even more stringent than they were in OTL. You probably have a Klan-type organization rise up to rule through terror via random killings and mutilations, with the law looking the other way. What won't happen is the surviving Southern whites suddenly saying, "Oh, I see, slavery is wrong and we brought all this on ourselves." The blame will go to John Brown and the people who financed him...the abolitionists. And the average Northerner will agree with them.