If you want to use real ribbon, I have the best results using double face satin ribbon. Put it on at the last minute so as to minimize the grease stains that may occur. This is going to depend on the quality of the ribbon (stains show up on cheaper ribbon faster). I’ve heard some people double up on the ribbon - the one next to the buttercream absorbs all the grease and the second layer doesn’t, so it doesn’t show.
My preference is to use ribbon on a fondant-covered cake. I do this all the time when I want a clean look to the bottom edge of the cake, no piping or bottom border to interrupt the design flow.
For your black ribbon, I’d say do it with black fondant and you’ll be happy with the results. Or even black modeling chocolate - the modeling chocolate is especially useful if the ribbon has to be attached to the top or centered on the cake. It won’t stick to itself the way fondant can.