Thermal paste for A8 chip
The article Thermal Paste Thermal Tape Question Logic Board mentions a similar black substance used in a G4 PowerBook logic board:


  When installing the logic board, I know your're required to use the silver colored thermal paste on the main processor and it's heat sink. On the other chip however, ( I assume it's the video chip, it says Motorola on it), the original techs appear to have used something black, and it is thicker, and harder to remove and appears to be cut to size. My question is,...what is the black stuff used on the other processor with a copper heat sink? Is it black thermal tape, cut to size and if so, where do you buy it from? And do you apply a piece of tape on the video processor and on the copper heat sink? Or do you apply the thermal substance on only only one part either the video chip or the copper heat sink?


Unfortunately, as you mention, there appears to be no source for the black thermal substance1. However, silver (which has an extra 5% efficiency) thermal paste, rather than white , should do the trick.



1 Unless this Thermal paste for iPhone 5 Mainboard is the black stuff.