Could a successful conquest of Egypt during the Crusades (either by the Kingdom of Jerusalem in the 1160s or by Crusaders while on their way to Jerusalem during one of the next Crusades) pave the way for further exploration into Southern regions of Africa and lead to the foundation of new Christian states in the Middle Ages, with the descendants of European-born rulers eventually marrying into native families?
The problem is that these populations mixing does not make a lot of economic sense. Egypt have in general been centered around the northern part of the Nile. The reason why I suggested America as a better place for this to happen, is that it’s one of the few places where overlapping white and black populations makes geographic sense. We can see it in the American south where the Black Belt border the white dominated highlands, or the mixed race Caribbean which is close to the white dominated Eastern Seaboard.
We could easily imagine if slaves arrived in the Caribbean dominated by city states in the 15th century, that free black or biracial people would join the non-noble upper class less than a century later, and continue to work themselves up, especially if the city states shift from city states to monarchies that the royalty will have significant African admixture in the 18th century, when a princess from the kingdom of Soleye (sun island OTL Cuba) marries the crown prince of the kingdom of Virginien (named after the Holy Virgin Mary and for some reason lying the same place as OTL Virginia).