Pete, same happened to my compressor. I decided to drain the compressor, expecting half a cup. Must have been about 3 or 4 pints of black sludge. I should have known better, as the humidity never drops below 70 and is usually closer to the 90's. Lesson learned.
I never thought of the compressor as the air has been pretty dry here for the last 10 years, but it has been raining here since Spring (September 2010) so in frustration I decided to drain the compressor. I have been using a small FleaBay airbrush compressor for a couple of years now with no problems, BUT--- I found the air tank, which would only hold about 3 pints of air, was probably 30% water (black water). I had to flush everything out, including the hoses, terrible mess- so take it from Ben's book , fit a water filter, I'm going to, it cost me hours of cursing with clear coats.
KC Dano has a good recipe for one here somewhere.