The 'blue' (more realistically, 'rainbow') colouration is caused by a strange refraction/relection of light in a VERY thin layer of oils on the tyre. The thickness is close to the wavelength of visible light, hence the odd interference colours. It's actually the same mechanism that make those very bright-blue butterlies so spectacular.... anyway.... I digress.
It's often associated with tyres being wet and thus, if you have left them somewhere damp over the Christmas break, that colouration can occur. It does show that oils are coming out of the tyre but..... I suppose it depends on HOW MUCH of the oils are coming out. I don't think it's often considered whether this is a good or bad thing. Do 1/10th of a lap and they'll be back to standard colour..... probably.
Ian
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