Black Mark makes the crew and the ship that crew is on cursed. The keyword states that any other crew on the ship retain their original nationalities. Would they be able to use their abilities once the ship becomes cursed or would some other ability need to be present to allow the other crew to use their abilities?
For example; Jacques duc de Valois (snape :D) is assigned to Le Bonaparte. Black Mark is declared to be in use. Jacques and the Bonaparte are now Cursed. The captain and helmsman that were also on the ship stay French. Can those two crew still use their abilities?
The ship becomes fully Cursed while the BM crewmember is on board, so crew matching the ship’s original nationality will not be able to use their abilities unless something else allows them.
In your example, if you need the ship to be Cursed, you’ll have to replace the captain and helmsman with Cursed versions if you want to use them. The French ones will only work if you use Snape without activating Black Mark.