He'll have to ask the card co what happened. African card holders buy online things every day they don't get blocked so his domain purchase shouldn't have been rejected. He should ask the card co why they didn't let him buy the domain.
He didn't give detailed explanations on the process he took trying to buy the domain and the kind of card he used or which provider he was dealing with, but we get the message he was trying to pass. Bitcoin extended the opportunity of doing business across international boarders without interference from government or requesting any layer of verification causing limitations. For any cross country deal with Bitcoin payment option, it is considered the best selective option.