Wow, these answers are all over the place, and that's the serious ones. I've had blueing on my track day tires, usually when I run on days when the temp is 90+ and the rubber's melting into little balls. I've run tires in that condition on one or two subsequent track days and never had any grip issues, but then I'm not necessarily the fastest guy out there, although I do drag my knee and don't have any chicken strips.
My two cents is that your tires are fine and you should be fine if you ride them until they're down to the wear bars. If you're after good handling, change them both at the same time. If you can live with some funkiness in the handling and want to save money, change the rear when it's worn and leave the front on until it's worn. Your choice.
