I'd definitely stay away from keeping a blue or unknown then.
I'd keep both black cockerels just in case you were to lose one or one not work out. Also you could evaluate them when older better then now.
Breed a black male to chocolate mottled girls. The cockerel offspring would all carry chocolate sight unseen so I wouldn't keep any.
All the pullet offspring would not carry chocolate but would carry mottling. I'd breed those back to the black males. Also a good reason to keep both.
With that generation you'd have nothing but black but the mottling gene would be hidden and some wouldn't have it.
You know those first generation pullets carry one gene for it so breed different males to just those for test breeding. Any males that produce any mottled chicks would have to carry a mottled gene also so just keep those. Then it's just a matter of producing as many chicks as you can to play the odds of getting mottle.