The ship becomes fully Cursed while the BM crewmember is on board
Does that imply that should the BM crew leave the ship, it would revert back to it’s original faction?
Yes.
The same thing happens to the ship* if Black Mark gets hit by a canceller. It’s not as devastating as when a point-reducer gets hit, but can still mess up your plans at least temporarily. (BM reactivates and re-Curses the ship* when the cancel wears off.)
*And the BM crew itself
The captive becomes “cargo worth … gold”, but doesn’t actually convert it into treasure, so gold modifiers won’t apply.
Ah shucks. I take it then other abilities that modify gold wouldn’t work as well, such as UT effects like Jade, Spices, Smuggled Goods, etc.?
Correct.