The black lead coils are just that a simple ignition coil.
The red lead coils have some of the electronics located in the coil. There is a transistor, capacitor and I beleive an SCR located in the red lead coils while the black lead systems have those components located in the stator.
I would assume (there is that word) these components where moved to the coil for better cooling.
The coils are interchangable in so far as you can use a red lead coil on all systems but a black lead coil is only suitable for a system with the electronic components located in the stator as without those conponents the system will not work.
Unfortunately I do not believe that if the transistor, capacitor and SCR fails in the 'black lead' stator you can not just replace the black lead coil with a red lead coil. Life is never that simple :'(
