Thank you...from all the things that I HAVE read, I didnt think it was possible either! Now I know Im not losing my mindA French Black Copper Marans x Blue Ameraucana could not possibly yield a splash chick... splash requires two dilution genes, and one would have to come from each parent. Genetically that mix should only give you blacks and blues.
What could be happening in your chick is a hidden gene, a sport. This can be some sort of recessive mutation that was sitting in the background, just waiting for the perfect situation. It will be really interesting to see this chick grow up!

Will update with pics as she changes!