A friend of mine just asked "so one of my friends got brown cage free eggs at the store I think. They boiled them and only one of the eggs shell turned bright blue, what can cause this?" Could anyone answer that for us?
I'm only guessing, given what little I understand of the egg-making process inside a hen's body.
That one blue egg may have been from a layer that had some Ameraucana genes. All eggs all either blue or white as they originate. During the final stages, the "tinting" process occurs where white eggs receive their "dye job" and turn the various shades and hues of tans, pinks, vanillas, and on up to the darker shades of brown and red.
The Ameraucana breeds will tint their eggs various shades of blues and greens, and some are even closer to brown than green.
If a commercial layer had Ameraucana genes, it could have laid a brown tint down on a blue egg, making it appear as brown as her fellow layers. However, this tint is sometimes pretty fragile, easily washed off. I notice this with my Welsummers. If I try to scrub an egg to get some poop off, the color will rub off, too.