I have a 2005. when i changed to fluid it was black as well. then changed it on a short interval and it was very dark. later on i had to replace the entire transfer case. the bushing had come loose and was spinning around inside , full of metal fillings and such. if it is very dark i would check it out. cost me over $2000 to get it rebuilt, should have just bought a rebuilt one, but did not reliaze all that was poched on it.
2005 Silverado 2500HD CC 4X4 5-speed Allison KD transfer case upgrade Reflex Spray in Bed coating, Access tonneau cover, Banks Ottomind 6 Tuner with Power PDA, Banks Monster exhaust,Banks Ram air intake. Ultrastart remote starter,Blistein shocks,Mag-hytech deep trans pan, Pulling 1982 27.5 ft Golden Falcon TT 275,000 kms