Two things spring to mind.
1. They're might be painted, not anodised.
and........
2. The colour was stripped off by the Oven Pride but it was left in to soak too long and has started to eat into the alloy giving it a black look.
If it's the first thing, then maybe some sort of paint stripper might be more effective (probably not the new Nitromors though).
If it's the second thing, it is easier as the black finish will polish off with a bit of elbow grease.