The Blackberry native API is pretty difficult and you have to handcode to CLDC (J2ME) which means limiting yourself to a subset of Java 1.3. The native widgets in RIM devices don't allow you to customize basic things like colors etc without changing the whole phone, this might have changed with very recent versions (haven't kept up) but regardless their "native" experience isn't a panacea.
Don't get me started on their IDE/integration's and simulator environments which are practically unusable.
LWUIT is effectively dead especially for the Blackberry. Your only real option is Codename One is maintained, allows Java 5 syntax and has a GUI builder. Its native in the sense that Java is generated and compiled properly but the UI is drawn by Codename One so the code is very portable.