Try looking at Sun Java's J2ME documentation
. A lot of it applies to the BlackBerry, as the BlackBerry is able to run midlets (including those originally written for Nokia, Motorola, SonyEricsson phones).
As far as I know, BlackBerry has no true out-of-the-box filesystem API to speak of, however the information tells you of methods of storing data on a J2ME device, which you can simulate a filesystem off.
Also, Dynoplex also provides you with a filesystem API if you would like a third party solution. It's called eFile