Personally, I view the black smokey stuff as an indication or symbolic of the tulpa's lack of a "soul." Remember the child that Richard ran over? Carl witnessed a golden light (soul) drifting out of the boy and upward. The golden pearl seeds that are eventually excised from the black smokey stuff are material and can be used to seed other tulpas. So, in a sense, gold indicates death and also artificial rebirth. That's my two cents. 

 Edit: Black Fire: Hawk likened it to modern day electricity but stated that it's the intention behind the "fire" that deems it black (or white presumably). Mr C may have used his DNA in the creation of both Diane and Dougie, so there could be something more to the black smokey stuff.