For me the red eye is usually not in the blue area, so you just desaturate the pupil and then darken if it you need to. If it was in the blue area too I suppose I would select the red and adjust the hue to some shade of blue and then desaturate that down to a more appropriate shade. If there was no pupil then I would just use the Dodge Tool… although you could end up with some weird crossed eyed effect if you didn’t pay careful attention 🙂
Alternatively you could just paint the color in with a really light opacity after you desaturate the red out, and then blend it together with the Blur Tool. That’s what I would try anyway. I hope this helps!