The quote in question from 1911 being: "Bunkum, friend Connolly; you are obsessed with an antipathy to Belfast and the black North..."

And here's another example from the same period (from a verse by Chesterton):

"The folk that live in black Belfast, their heart is in their mouth,
They see us making murders in the meadows of the South;
They think the plough's a rack, they do, and cattle calls are creeds,
And they think we're burnin' witches, when we're only burning weeds."

(And in both these cases the "black" may in fact refer to the smoky industrialised part of the North, as in the Black Country in England.)