You should read this FAQ, It's an interesting read:
http://www.bmw2002faq.com/content/view/69/32/
It explains why the blue coil is actually a down-grade from the original coil that requires the external ballast resistor (or the resistor wire used on the 1974 and later cars). Basically, you loose the boost function of the coil (during starting) if you set up the blue coil properly. Blue coils work fine as long as you bypass the ballast circuit correctly. On some cars, like tiis, you might want that boost (extra spark) when starting. Best to leave it as BMW designed it. I've found that all of my original "Black" coils are the same as the "Red" ones sold today. My black ones have stickers on them saying that they require the 1.8 Ohm ballast resistor. I guess some 2002's came with Black coils that required a different 0.9 resistor, so check what you've got. Your resistor should be marked on it's metal base with either a 1,8 or a 0,9.