There are 3 possible reasons for the blue egg from a normally brown or dark colored egg laying hen.Buttons and yes i was surprised the first time i got these too here i have old pics that old shipment only one celadon survived the mail but here u go it was a pretty blue and all the other eggs are celadon on the inside
1. The paint department had a malfunction.
2. The hen has reached the end of her egg laying for the season, although buttons usually will lay year round, even if not given artificial lighting.
3. The hen is at the end of her reproductive/ egg laying life.