I heard Alex playing this simple tune at a session and he passed on the notes to me. It is quite fun to play and not hard to learn.
The Black Joak or Joke or Jock turns out to have a long history (and even a Wikipedia entry). It was a bawdy song in the early 18th century. Recorded in John Walsh, Third Book of the Most Celebrated Jiggs, Lancashire Hornpipes, Scotch and Highland Lilts, Northern Frisks published in 1731. The publication did not record the words, but versions of them have been recorded elsewhere, and you can read about the essential idea on Pete Cooper’s notes or listen to the Mother of All Morris singing the Black Joke being sung on youtube.
The Scottish version of the tune changed the name to make it less sexual, which tells you something about 18th century attitudes, but the tune is still there. And perhaps there is another joke in the tune, because the B part is four bars longer than the A part, so it can trip you up! The bawdy song was popular, however, and when the Brazilian slaving ship, Henriquetta was captured in 1827, it was renamed HMS Black Joke and spent the next five years chasing and capturing many other slave ships and freeing slaves.