Mine's doing this too, it's the master cylinder on mine. There's a tiny leak inside the car above the clutch pedal, but because air got into the system it's doing exactly what you're describing.

There's a chance it's your fulcrum ball or the throwout fork broke, but I can pretty much guarantee you it's a hydraulic issue. Especially since your clutch fluid is nasty black, its probably been wearing down your rubber seals with all that grime in it.

This is a good thing since the hydraulic system is super simple. Just a master cylinder on one side with a reservoir, a couple lines easily visible, and a slave cylinder.