Why does some greasy black powder keep appearing on the bottom of my transmission pan (outside)?

------------------
87 Iroc-Z LB9
Dynomax exhaust
modified MAF & airbox
3-10" JL subs
Eclipse components

 

You've got a slow leak that is collecting dust and evaporating the light stuff to form what appears to be a greasy goo.

------------------
82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R. 2.93 limited slip. Cat-back from '91 GTA, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LG4 w/'87 LB9 block, ZZ3 cam and intake, World 305 heads, Hooker headers & y-pipe, 3" Catco cat).
57 Bel Air, my 1st car. 0.030 over 396, Weiand Action+, Edelbrock 1901 Q-Jet, Jacobs Omnipack, 1-3/4" Hedders, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Best 15.1 @ 5800' Bandimere w/open diff & slipping tranny. Daily driver while Camaro was being put together.

 

where would it be leaking from? No leaks anywhere that i can see. if i had to guess it would be between the tranny pan and the tranny. is there a gasket there?