My Guzzi pipes haven't blued because they're black but both my Ducatis have the blues. I polished the headers as they were initially a dull, almost rusty colour and they started turning blue within a few minutes of running, now they have a nice graduation from blue to gold and lots inbetween.
I like it, makes it look used, this is what the Monster's pipes look like;
For the record, this bike runs on the rich side rather than lean but it does run quite hot.