My 2015 M6 Sportnav had the same dark alloy peeling/blistering issue about 18 months after purchase. I went back to the dealer and there was no quibble whatsoever. It is a well known issue internally, something to do with the incompatibility of the Lacquer and the road salt we use over our winter months. I had all four wheels replaced, although I still believe the replacement wheels are not quite as dark as my originals were. The dealer explained this (and showed me) that the internal document which describes the known issue with the blistering also describes a darkening of the original wheels over time due to dark spots under the lacquer (like the old newspaper picture dots) which creates an illusion of a darker wheel when compared side-by-side with the replacements. I have had the replacements for a year now and no issues so far (fingers crossed), even through our beast from the east weather this winter. One thing to note though was that the Dealer said to me that if my wheels had been scuffed/kerbed they would have void the warranty, fortunately for me I am a careful when parking and mine were pristine, so they had no way out on that one.