It could be surprisingly difficult to find items discussing racism (or lack of it) in modernist magazines. Interestingly, it was also more difficult to find articles centered around black people than I anticipated; I found a lot more articles on other races. I found a decent amount of material in The Masses and The New Age, however. Both Volume 1, No. 7 of The New Age (article "A New Outlook in India") and Volume 6, No. 3 of The Masses ("Race Nonsense Again") had articles against racism.
Both A New Outlook in India and Race Nonsense Again use a mixture of pathos, logos, and ethos. A New Outlook in India argues its case by using mostly logos and a little pathos, throwing out enough facts, figures, and names to overwhelm the reader. It also tries to make the reader feel pity for the Indians under Britain's rule by talking about how hopeless it's gotten, and makes its point (that the Secretary of State is to blame) by directing the audience's frustration and anger at them. Race Nonsense Again uses mainly pathos and a little ethos. The author tries inciting the audience by talking about the slurs and insults used against the "negroid" black Portuguese. The author also uses ethos by discrediting and insulting their opponent, implying that the generalizations made in the article show that the author of the article they're talking about is incompetent.