We got to the question because he was first wondering why there weren't more blacks in Arizona, considering that cotton has historically been such a big industry here.
I sort of just stared blankly at him for that.
It's called the Second Great Migration and it started after the U.S. entered World War II. I think this counts as general knowledge on this sub, but here is an essay.
Basically, the US was making ships and planes and other armaments on the West Coast and the factories and shipyards were hiring African-Americans and paying well. California lacked Jim Crow laws, so it was a real opportunity for economic and social mobility.