I’m going to attempt to answer my own question:
After digging through the core code for NVIDIA Blast, I found that the Blast Asset, which is the destructible mesh, is put together in a very specific way. This asset contains a skeletal mesh with 2 material elements, exterior and interior, and each element is basically a separate mesh bound to the same skeleton. Then Blast works out which parts to show and which parts to hide. Because of this, I can only assign 1 material to the entirety of the interior mesh. This makes it impossible to assign a unique material instance to an individual chunk of debris. Please, anyone let me know if I have missed something.
If anyone can think of a work around to make the individual debris chunks appear to have separate materials please chime in. I was thinking perhaps a post process effect but that’s my absolute last resort.