Darker blue eggs
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I know that the "blue" egged birds will sometimes throw green. So, I think if you try mixing for an olive egger (dark egged bird/black copper maran over blue egger) and then mixing back out of it, you could possibily produce a darker blue. I will say I am not up on all the genetic gargons and workings, but it would be a way I'd attack it.
Edited by Kathyb2 - 3/3/16 at 7:15pm