I also am not sure what you are calling black iron. A lot of cast iron drain pipe is "black iron" and wont be welded with a 6010 rod. You might get by with a 7018 if it doesnt crack after welding but a nickel based rod specifically for cast iron is what you would need to use.
Regular steel pipe (ASTM A 53/106) can be welded with any of the 60xx rods very easily. For DC welding, use the 6010 for the root pass (you can weld it all the way with these rods also) then switch to a 7018 for fill and cap. This gives you the strongest weld.