until you start talking about salvage where you can't take any other pieces of gear for granted at all (except the other earring and two rings, in this case)

but yeah, generally speaking a blackmage will have enough MAB that they'll be right to wear a static over a MAB earring. (and yeah, I realize it's highly unlikely a blackmage would have magic and rings but not body cell lol)
it's still worth mentioning if only because blackmages (while the subject of this thread) aren't the only ones who MB and might end up needing to pick item choices (it's relevant for the case of novio+static vs novio+moldavite too for redmage, for instance.)