I've done a little reading and it's been recommended in a thread over on TFW to use a gloss or satin clear coat from a hobby shop that is used for miniatures.

is it metallic paint or chrome that's wearing.

the 'scientific' explanation is probably some form of oxidation of the paint. depending on what they've used to actually create the blue colour, humidity, heat or just light, might have an effect.

I know years ago I bought my then girlfriend, now wife a blue topaz ring. several years later it was practically clear, and crappy looking. on researching that, it turns out that a lot of coloured topaz is treated with radiation of a particular wavelength to artificially create a colour in it. This treatment is not a permanent treatment and exposure to UV light weakens the colour over time. needless to say I'm pretty pissed and will never be back at that jeweller.