About a 50/50 split of birds being black and birds being blue in color. Splash of wheaton showing through, amount will vary bird to bird.
There is a lot more going on with isabelle color but basically it's lavender base. Lavender is not blue and will behave like black when crossed to Andalusion Blue which the Isbar are. Hence 50% black and 50% blue chicks. All should be split for lavender (self blue) color.