If the oil is black, it's because the seals have deteriorated and contaminated the fluid. You will have to replace the clutch master cylinder. While you're at it you might as well replace the slave cyclinder too. You can verify this by having someone press down on the clutch pedal while you observe the arm on the trans, if it doesn't move at least 3/4 of an inch then your master and slave are probably junk.

 

I am assuming that you have a 4 speed manual trans? Look into replacing the master and slave cylinders for the clutch, after sitting and the hydraulic fluid absorbing moisture its not uncommon for these to go bad.

 

if none of those work you're going to have to take the tranny out and make sure the clutch assembly is working. it could be something as simple as your clutch fork slipping off the pivot point inside the transmission or your throw out bearing could be shot.

 

Years ago the tranny in my 76 2+2 didn't want to shift, I used a mixture of 6 parts 90wt gear lube and 1 part type F automatic tranny fluid and that resolved the shifting problem. Still running the same tranny and after many, many miles its still smooth as silk. When ever I service the trans I use the same mixture.