If you put a black rooster over your lavender hens, all of the offspring should be splits, visually black but carrying one copy of the lavender gene. Then you can breed the offspring to each other and you should statistically get 25 % lavender from this crossing. The other 75 % would be splits and pure black with no copies of lavender. You can't tell the splits and blacks apart without test breeding which is time consuming and requires more space. Or you could just raise them and sell them as layers/roosters for a backyard flock (most likely not a breeding stock since you don't know who is carrying what).
Alternatively, you could breed the same splits to a lavender bird and get 50 % (again statistically) lavender, BUT I would not do this unless you could get your hands on a lavender that is at least as nice as your nicest splits. You wouldn't want to use your original lavenders because you would probably be digressing. In this case, the 50 % that are visually black would all be splits.
I hope this helps ... how are your pullets doing?
Edited to make the change highlighted by WA4-Hpoultrymom Ha! Thanks for catching my mistake! You are right of course ... I should have proofread! :)
Edited by cukooformarans - 3/19/12 at 5:33pm