Thank you! So breeding this pullet to my mottled OEGB cockerel will result in solid black offspring. Then breeding the offspring to each other might possibly result in some lavender mottled?
Yes.
In that second generation, you're likely to get about 1/4 lavender chicks, and about 1/4 mottled chicks.
Of those, about 1/16 of the total chicks "should" be both lavender AND mottled (1/4 x 1/4).
But of course those are averages that only work right when applied to large numbers of chicks. If you hatch exactly 16 chicks, you probably will not get a perfect distribution of all the possibilities.