My two cents is thinking you either have a female black orpington or female australorp (which essentially is the same thing, their lineage comes from black orps).
By 12 weeks old, you should be able to tell gender. The comb and wattles are small on this bird at 12 weeks. Her legs are also thinner that i can tell from the pictures. Boys often tend to have stockier, thicker legs.
So i believe you have a hen and no matter which one of those two breeds she is, she'll lay you some nice big eggs when she's ready.