I've tried it both ways - building a pure MIDP app to run on BlackBerry and non-BLackBerry platforms and building a separate BlackBerry app (often using much of the same business-logic and networking code as the MIDP app). Definitely go the BlackBerry native route.
It's all about the BB UI classes. They'll give you the ability to (among other things) respond to the different type of menu events (trackball and menu key), respond to BB specific key codes, if you're interested in the Storm take advantage of the orientation sensor and touch support. Plus they're a much richer set of UI elements to work with. You can build up a lot (but not all) of what they do in pure MIDP, but end up customizing so much of it for each platform that you won't save anything in the end. Starting with the BB UI and customizing saves a lot of time and effort.
Even in gaming applications, or for applications where you're custom drawing all your components, you have better access to the BlackBerry graphics APIs and get better performance going the BlackBerry native app route. And you still have better detail about input events.
Finally there are some nice lifecycle things you can do with BlackBerry native apps, like pushing to the background, or auto-running on system startup that you can't do with MIDlets, which may be of interest depending on your application.
Also think about market - if you're planning on supporting BlackBerry right away, and then maybe other MIDP platforms down the road, it's usually a better plan to execute the best you can on your initial platform. There probably won't be much of a reason to port to MIDP later if you don't succeed on BlackBerry first.
As far as I know, there are no Basic or C interpreters or IDEs that run specifically on the BlackBerry, but there is an open source J2ME basic dialect called CellularBasic which is meant to be coded with/executed on the device itself. It targets J2ME, so it may run on the BlackBerry: http://cellbasic.sourceforge.net/
Best Answer
I developed a standard J2ME application in netbeans with about 10 UI screens and using webservices. I was quiet satisfied with the results. I then spent about a week trying to "port" this application to a blackberry. (Trying your approach of getting blackberry api working on Netbeans and trying native J2ME on the blackberry SDK)
I then only managed to get successful results by starting a fresh Blackberry application using the standard blackberry Eclipse IDE. Although I have two separate applications in different platforms I am happy with this two IDE approach.