In The Color Purple how is it a black person can have so much wealth in the south?

It is possible that he earned or stole his wealth, but many times more likely that he inherited it.

The earlier the scenes with the rich Mr. Albert Johnson were set, the more likely he was to have inherited his wealth from a white ancestor. And I think that even in 1940 it would have been very hard for an African American in the south to have earned or stolen enough money to become a plantation owner.

In that era most southern states had laws restricting the rights of African Americans or blacks. And they had "single drop of blood" laws saying that if someone had even a single known black ancestor many generations earlier he was legally totally black. And often laws that if someone had even a single Indian ancestor many generations back he was legally totally black. And they had anti miscegenation laws making sexual relations or marriage between whites and blacks illegal.

So for generations partially black persons in the south were considered totally black and were only allowed to marry other partially or totally black persons. If someone was partially black in the south, his descendants would be legally black for generation after generation without hope of ever becoming white.

So if Mr. Albert Johnson in The Color Purple is considered black by everyone it would still be possible for him to be biologically part white in ancestry.

Before emancipation, male slave owners and their male relatives often had sexual relations with female slaves who had no legal right to resist. Thus many wealthy slave owners had illegitimate half black children. And some of the wealthy slave owners treated their half back illegitimate children as slaves the same as the rest of their slaves. Abolitionists clamed that some descendants of the Founding Fathers of America had been sold in slave auctions. And some of the wealthy slave owners treated their illegitimate half black children like they would treat their illegitimate white children, as family members, though lower in status than their legitimate white children.

Thus wealthy slave owners sometimes provided for their illegitimate half black children by giving them education, money, and landed property. So before the war there were some partially black wealthy slave and plantation owners in the South. And like the white plantation owners, many of them retained their plantations after the war and used sharecroppers instead of slaves to grow their crops, and bequeathed their plantations to their children.

So Mr. Albert Johnson in The Color Purple probably inherited his plantation from his ancestors like most white plantation owners, and his relationship with wealthy and influential white plantation owners who might have been as close as his first cousins probably protected him from trouble.