I was told that is due to the additives in the gas. Some say if you use super 93+ octane it doesn't do that as bad. 93 is all i use and my power pros chrome is not discolored at all.
Just thinking more about this: boat headers won't turn blue if you run water through them, but will turn blue when run dry.....so it must be the heat turning them blue.
You can use this stuff called Blue Job. Thats what I use on my Harley, comes right out. Heres the link, I 'm sure u can get it cheeper on ebay or another sight just by searching around. Blue-Job Chrome Polish, No-Scratch, Removes blueing and rust
If it's turning blue it means it's getting hot, which means you are running lean. What does your plug look like? Consider doing a plug chop or bumping one jet size