Wheel wells are a huge problem for our Tahoe - they get dirty instantly, and never look "black" for very long unless you use a dressing, which attracts dirt/dust...ahhh, the viscious circle.
Here's a trick that I found works well on our Tahoe. It's a little easier to do on that than on the BMW, but it works well.
1) Get a bottle of full strength Simple Green, and get an empty spray bottle (I like the PowerShot bottles from Lowes $2.95, 32oz).
2) Pour about 8oz. of SG into the empty bottle, add about 12-14 oz. of water.
3) With the car dry (before wash), spray the mixture all over the wheel wells. Saturate them.
4) Clean your rims and tires as usual.
5) Before you rinse your wheels/tires, take a stiff paint brush or a small brush of some sort (what ever will fit in there easily) and agitate the surfaces.
6) Using the strongest water stream possible, rinse the heck out of the wheel wells.
When it dries, it should be black. This works on plastic and "undercoating".