One of them is for 'lights out machining', while the other can be safely used in the daylight
Seriously, I think it has to do with the amount of the sulfer compound additive. The dark stuff is what I've always used when tapping steels of any type. Seems to promote tap life somehow, I guess it prevents that 'chip welding' phenomena that wants to break off the sharp tips of the tap teeth. This effect then produces cleaner looking threads. I think the guys who run pipe threaders all use the dark stuff, too.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)