You're talking about Brake pad dust and tar buildup.
As Llew as indicated APC or claying.
What i have found is that, it takes many many applications of a proper wheel cleaner (the kind that turns color when applied and reacts with the metal/carbon). i use Sonax wheel cleaner and even on the most resistant wheels, about 3-4 cycles of apply, wait, agitate, rinse should get most, if not all of it off. Anything that remains, follow with a metal remover, available from forum detailer/resellers. Rinse and repeat.
Then clay the wheels, rinse and then wash again with apc, then hand dry and wax.
In my own experience this is best done in the shade, so the chemicals dont dry before they can work and best results achieved with the wheels off the car.
This means its a full day job if done yourself to jack, jack stand, remove, clean, dry, wax and install wheels again.
If the wheels have never ever been cleaned properly, i have had this, meaning you have years of dust and tar buildup, bitumen, oil and brake dust as well, it will take much more time. I found this irritating enough for me, because OCD, to remove wheels and clean them over 2 days, starting on Friday evening by removing wheels and preparing everything, pressure washer, hosepipe, brushware etc.
Saturday wake up early, wash, wash, wash, wash. eat lunch with brake dust under your fingernails, wash wash wash.
Sunday, wash, wash, dry, clay, reinstall wheels.
Cut nails, bath, admire your wheels. send stance pics to your mates and fellow enthusiasts.
Monday, your wife will threaten to leave you.