Ok kind of late i know.

I May be wrong, but yes, their respective duties were very similar. That bring said the National republican guard although clashing at times with allies troops had much less martial combat, being more a policing unit, focused more on things like retaliations, arrests, round ups and investigations on the field. The Black brigades instead were the very armed wing of the Fascist Republican Party and had usually a more "Active combat" duty, especially against partisans and Rebel groups in the mountains of northern Italy.