The Frederick Douglass quotation is important, but we should be careful about how we use it. For one, I don't believe Douglass was actually close enough to personally witness black Confederates in action. Thus, his account is at least second-hand, if not third-hand or more. Secondly, Douglass was trying to get the federal government to accept black military service. Emphasizing reports of the enemy using blacks as soldiers could have been a useful tool in convincing some white northerners that black soldiers could and should be put in the field for the Union.

So, just like with all historical sources and evidence, we should definitely recognize its value, but consider the varying factors behind how Douglass got this information and why he said it.