GM / General Motors: 92 Astro 4.3 Coolant loss

I lost about 2 quarts of coolant in the past 2 weeks (about 500 miles
of driving). There is no coolant leak on the ground under the vehicle
that I can see, hoses, connections and water pump are ok. Hard to see
the engine sides because it's a van, but no noticeable leakage there
or from radiator. Vehicle runs well no noticeable misses. When hood
is open, there is an antifreeze "smell" similar to when a hose breaks
or there is a leak. Not always but possibly every 3rd or 4th time - I
get a white puff of smoke when starting the vehicle. When I took off
the radiator cap, there was a black gooey substance around the
underneath of the cap. The reservoir also seems to be off color and
blackish, but the antifreeze in the radiator looks clean. I didn't
spot any antifreeze on the dipstick, or when changing oil. If it's a
head or head gasket, how can I tell which side it is? If it's the
head, the motor has about 180000 miles on it, am I going to have
trouble with new valves and oil consumption? Right now, the motor
doesn't use oil, or smoke.