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Black deposits in intake

Having removed my 3B intake manifold, I'm puzzled by the appearance of fine, black, granualar deposits which have formed (inside) on each side of each runner, continuing on into the ports.  Otherwise, the runners, ports, and valves are clean and shiny as you'd expect.   The stuff is coal-black, and almost crystaline, comes off easily when rubbed.  What could it be?  Something in fuel? Some kind of blow back from combustion? 
Living in fear of discovering something that will head me back toward a full rebuild,
Brandon Hull
'91 CQ+3B heart transplant