I've never caught a White Crappie with the black stripe. But, that being said ... we did have this posted back a few years ago : http://www.crappie.com/crappie/main-...crappie-2.html ... so it does occur.
And if you look at this thread, you can see that even the naturally occurring hybrid cross Crappie can end up with a black stripe : http://www.crappie.com/crappie/main-...crappie-2.html ... so it's not so unlikely that a hybrid cross Crappie could end up with the black stripe AND White Crappie markings.

The black stripe is just the results of a marker gene that occurs naturally in the wild in several different states. It IS, normally, associated with the Black Crappie ... but, because it's a gene caused mark, it can be transmitted via crossbreeding.