Black meat is caused by one or two things..(or both) Over Smoked or burnt sugar. The sugar can come from any number of sources...brines, rubs, fruit juices etc. as well as sugary sauces applied too early in the cooking process. Since the Loin Backs looked "OK" to you, I would suggest sugar, (too much?) as the problem, plus the fact that the spares were exposed to the heat longer than the loin backs...That slight difference in time caused the sugar to burn/darken on the spares...HTH!!!