Black second mates on sailing ships in the 1890's were extremely rare, in the context of when the poem was composed, and it was composed by a sailor-poet. That's why I'm satisified with Q, Giok and Uncle Dave's explanation.

Now I did say "rare" not "non-existent" (before someone else flies off the handle). There were even Black whaling captains, some with all black crews, in the 1820's as attested to in log books of the period. Barry Finn sings a great ballad commemorating one such captain, a Captain Boston.

That's not to say that the phrase in a different situation or at a later time didn't have a racial slur element, or a sexual one as I've mentioned above, and evidently it now has a culinary reference to Thai sateed food. Whatever!

Charley Noble