Dark fluid as reported = failed slave cylinder
(piston is spinning due to failed release bearing) no amount of flushing is going to fix. Too late.
When the slave is pulled there will be a dark sludge in the cavity and the slave end. It looks like grease but it is water soluble (product of brake fluid leakage) be sure to clean out completely.
BB has good price on slave cylinders that are the exact OEM Magura replacement.
Before installing be sure to check the new slave release bearing for adequate grease.
I find almost every new one from BMW has just a tiny dab of grease often not even touching the balls (automated greasing?)
Add high temp wheel bearing grease with a Q tip and work around ensuring that all the balls are well greased.
That wee bearing is spinning all the time!
While in there be sure to cut back the neoprene sleeve that partially covers the pressure line coming down from the clutch lever.
It typically holds water in it and will eventually rust out the elbow above the banjo. Cut back about 1"
As far as bleeding the system goes I push the brake fluid up from the bottom the way BMW does it at the factory.
A modified brake bleed screw (pushes the ball) and a syringe does the job. Some find bleeding a dry clutch system frustrating, this method takes 2 minutes.
Cover the open clutch reservoir with a rag as fluid will shoot up when it arrives!
Do not over fill, as the clutch wears the reservoir level rises (opposite of the brake system)
Note: I believe that greasing the release bearing is as important as regularly flushing brake fluid.
original slave cylinder is at 254,000km. I change my fluid every year and have re-lubed the bearing 3 times.
Notice the dry balls in upper picture, this is typical of what I have seen.
Detail on how to trim the neoprene sleeve. This is a near new cable and the rust is just starting. Most times rust has progressed much further by the time I see them
Reverse bleed, also works great when installing SS Brake lines
Closeup of modified bleed screw (available at any parts jobber) Grind down the end so the threads will engage, turn in and the tip will push the ball back so the fluid can go by.