Either way but black plugs can be from more than idle mixture. Simply short trips or short running time without getting things warmed can cause that, or "big cam" (Not all that big) can cause other circuits in the carb to run rich.
Once you find the peak "lean" towards the lean side, just at the point where vacuum or RPM "just not quite starts" to fall off, in other words, "lean towards lean" rather than towards rich side of the peak
It is IMPORTANT that engine be fully warmed. If it is cool in your area, some cardboard partially blocking the radiator, temporarily might do it