In 1861 Frederick Douglass was a journalist and newspaper/magazine publisher. Once the war started he served as a recruiter of black soldiers for the Union Army. I’m guessing he had many sources throughout the U.S. and the confederacy.
I have a lot of respect for Frederick Douglass. He was a major bad-ass.
The Union didn't officially enlist blacks until after the Emancipation Proclamation. The quote in question was made by Douglas in September of 1861, when he had no way of knowing who the Confederate were enlisting. But it was a good story to shame the Lincoln administration into enlisting blacks themselves.