It's my understanding some lines of black copper Marans harbor recessive Wheaten genetics, so the random yellow chick isn't unheard of. Not especially desired when breeding pure black coppers, but since you're dealing with a probable mix it's no biggie.
Sounds right to me.
When I sell mixed chicks, I write down what I think the mix is, but always with the disclaimer I may be wrong. Most folks who are buying mixed aren't that particular. They like to know, but understand the guessing is half the fun