Any suggestions as to why/how? You can just see a blue ring where the tread and sidewall meet. I know the tires are pretty old and I'll be getting a new set soon.

If not just take a look in amusement.
http://www.vilipend.com/~frrax/album18

I know my RA1s get a little bluish discoloration around that same area when they're very old. I think its heat discoloration. If you don't track those tires though, it might be a different story.

The car isn't driven much at all, mostly for autocross. It has been sitting for a while during the motor swap, under a blue cover. The rear has a mind of it's own most of the time but I didn't think rubber would change colors from heat... metal yes, rubber no!!

I've seen the same thing on brand new Hoosiers and Nittos after being in direct sunlight for less than an hour.

So Sean, you're saying this color change is due to sunlight? So...is it bad? I've never seen it on my street cars, just the RA1s on the Camaro.

I've already said about all I know on the subject. I assume the heat or UV is cooking something out of the rubber.

Since I use an open trailer for my tires, I've started waiting until late in the day before an event to load the trailer. At the event itself, the tread is mostly protected from the sun during midday by the fenders, though the sidewalls will get some sun.