Oda said Blackbeard is Somalian which is in Africa. Usopp is also African but undisclosed which part of it by Oda, I suspect South Africa though potentially.
Marshall D Usopp has a nice ring to it too, doesn't it? Usopps full name is unknown as well as the full name of both of his parents and I suspect his parents are a Montblanc and Marshall potentially and thus Usopp is Nolands descendant.
Marshall can refer to Sheriffs/Deputies and such too, the Wild West, gunslingers, "Judges" and such. Usopp and Yasopp are snipers and both Blackbeard and especially Shanks have gunslingers in their crew....
Plus the blatant similar appearances as you showed there ofc! Like Brook and Aokiji!