bimmer looks fabulous.
Why is it that people who OWN such great cars don't even have rudimentary knowledge of how to wash & maintain a car's surface?
Sounds like tons of baked-on brake dust, think I read somewhere that BMW's OEM pads shed lots of brake dust. Remedied this situation on my Accord by switching to ceramic pads and brake dust is somewhere between negligible & nill while maintaining excellent braking performance.
I suggest a wheel cleaner and try not to get it on the face of the wheel. Rinse frequently if you do. Use a green scotchbrite pad or even some 000 or 00 steel wool on that baked on brake dust.
If you referring to the area between the spokes, avoid previous suggestions about scotchbrite pad & steel wool but you're still going to need some kind of chemical cleaner.
Poorboy's is releasing Spray & Rinse Wheel Cleaner soon, definitely worth a try based on reviews of test samples.