The eyes Might have it...
Hyper-Keratin
My guess is the zombies are hardened by a hyper production of Keratin in skin and bones. Great for deflecting bullets, but it might really suck in the eyes. Might screw them up and make it difficult to see. The Blue / green eyes do not produce the melanin. So, if I was super creative, I might say that they same ressesive trait that prevents the production of melanin, might also reduce the amount of hyper-keratin produced.
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