Checked a used car today

hello every one i looked at a 89 beretta GT today for my 16 year old, and every thing was fine good exterior interior ran strong, i only have one question thought. when i looked at the tranny fluid it was black... and then 20 mins after test driving it i looked at it again and there was non in there on the dip stick...what do you think the problem is thanks for your time

Time for a new transmission. Black fluid is from overheating of the transmission, which can be caused either by slippage or by a bad torque converter. If the fluid can't be seen on the stick when the engine is running, then this is a cause of slippage, which in turn turns the fluid black. And burns the clutches, necessitating a new transmission.