It's probably from your green layer. Green eggs are just blue eggs with brown spray paint added on. So this one could have skipped the paint booth and come out blue. Congrats though, it's pretty!
I think you have a late bloomer. In our first flock we have two EE's and I thought that one of them was laying a light brownish egg. At about 10 1/2 months, we suddenly started getting a light olive egg in addition to the light blue/green one we had been getting--late bloomer number one. At about a year, our second Barred Rock started laying (we had thought possible closet roo), and our Welsummer started laying a very beautiful Terra Cotta egg---late bloomers number two and three. There just is the occasional bird that matures late. I think you may have one.
EE's are famously late bloomers. And my understanding is that hens have a color of egg they lay. Not some days it's blue some days green. A blue egg layer will always lay blue eggs. After they molt the color may be more intense but it will not be a different color.
Ahem. But your egg is beautiful, check the girls ears my piggy has sky blue ear lobes and thus far is my only blue egger. Her daughter, well her eldest daughter, is laying me a dark pink egg. Piggy was nearly a year old when she started laying and her eldest daughter is nearing 7 months, she's laying but her hatch mates aren't. Might just be a late bloomer.