I don't know how good the lavender ameraucanas are these days. I am assuming the blacks are better & typier than the lavs. If you cross your best lavender to a top notch unrelated black, yes you will get black split but you'll also be introducing good qualities from the black lines.
Personally I'd breed quality black male to best lavender pullets (pen A), plus best lavender male to quality black females (pen B) . From there if you can breed males of one pen to the females of the other & select for lavender & best type, feathering etc. offspring. I think that would probably have the fastest improvement in type (& hopefully feather quality). I think that is probably the quickest & easiest way to improve lavenders. Alternatively you could cross back the best female black split offspring from pen A to the lavender male of pen B & the best male split offspring from pen B to the lavender females of pen A. then you will have more lavenders to selet between but a greater chance of the offspring inheriting traits you might be trying to breed away from.