My guess is that it is from one of your new layers, and that it is a double-yolker. Have you cracked it yet? I had a few chickens that laid a LOT of double-yolkers when they were first getting started, and they looked just like full size eggs.
You said you're sure it was from your blue egg layer because you'd just gathered eggs from all 4 of your other chickens? Is it possible one of your brown egg-layers could have laid late the day before? I agree with the others that it's not likely that a chicken would switch from blue eggs to brown, probably more likely (though also rare) for a new layer to lay twice in one day. My chickens will occasionally sing the egg song for one another and go in and out of the nest boxes without laying, so maybe she was just bluffing?