Some digging has led me to one purported source, Arthur de Gobineau's "Essay on the Inequality of the Human Races". A cursory skim of the document reveals it to be about what you'd expect from an essay that introduced "Aryan" as a term for Germanic people. While the term "Negroes of the north" does not directly appear, there are a few hints of this. First, his definition of the "three races" is
I find these races naturally divided into three, and three only - the white, the black, and the yellow. [...] by yellow [I mean] the Altaic, Mongol, Finnish, and Tatar branches.
Of which he later says
As for the black and yellow types, they are mere savages in the tertiary stage, and have no history at all.
Touching on the Finns, he presents his opponents' theory and his rebuttal for it as such:
The unitarian theory is backed by such arguments as the following: "The Magyars are of Finnish origin, and allied to the Laplanders, Samoyedes, and Eskimos. These are all people of low stature, with wide faces and prominent cheek-bones, yellowish or dirty brown in colour. The Magyars, however, are tall and well set up; their limbs are long, supple and vigorous, their features are of marked beauty, and resemble those of the white nations. The Finns have always been weak, unintelligent, and oppressed. The Magyars take a high place among the conquerors of the world. They have enslaved others, but have never been slaves themselves. Thus, since the Magyars are Finns, and are so different, physically and morally, from all the other branches of their primitive stock, they must have changed enormously."
[...]
[...] Happily for common sense, there is no need for surprise, as the argument, though otherwise perfect, makes one vital mistake—the Hungarians are certainly not Finns.
Some time later, de Gobineau discusses mixed races, and concludes
[...]Again, in the Malayan variety, a human family was produced from the yellow and black races that had more intelligence than either of its ancestors. Finally, from the union of white and yellow, certain intermediary peoples have sprung, who are superior to the purely Finnish tribes as well as to the negroes.
So, I would say that it is credible that the characterization "Negroes of the North" has been applied to Finns.