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Mottled X Black = 100% Black offspring, recessively carrying the Mottled gene. These birds might have "spots" when they are young, but will loose most, if not all when they mature.
Breed the offspring together, F1 X F1, OR breed the F1 to the Mottled parent bird and in your F2 generation you will produce visually Mottled offspring. These birds can be considered pure Mottled and when bred together will produce 100% Mottled progeny.