I don't think the Mexican gangs 'took over' the black neighborhoods. In a neighborhood where the vast majority is African-American, a Mexican minority won't have a chance.
The Aryan Brotherhood and La Eme have an alliance because they have mutual enemies. They also do some business together like working in the drug trade, etc...
Black gang aren't really big in the Texan criminal world. It's mostly Chicanos and Mexican immigrants and a lot of whites doing the dirty stuff over there. Arizona is also mostly controlled by outlaw motorcycle gangs and Hispanic gangs.