Did you happen to get your BCM's from Wynette? Because I know she had a BCM girl that would occasionally lay a purple egg. And she described it the same way you did, when you wet the egg, it turned normal brown color, when it dried, back to purple.
I have noticed this type of partial coloring on eggs before but never to this rich, dark purple. You haven't been giving them any grape Kool-Aid lately have you? LOL Now if you can just come up with bright orange or yellow eggs out of your hens you will have the whole color spectrum!