I am currently subbing out any " transfers " for white text on black to vinyl heat transfers or DTG,which is more expensive,especially on dark fabrics. I usually use DTG on darks for multi color designs, because the hand or feel of the design is much softer. To answer your question,printers don't print white. You could also look up info on custom plastisol transfers.
Mike
It is possible, but you need to cut the letters out of the dark paper, either by hand or with a cutter. The dark paper is probably the least market viable product of all the transfer processed, but is used in retail by alot of people - if the market is right it can be done.
Something to think about, in addition to Mike's suggestion, which I would likely suggest you try vinyl for this as well, is that you can buy precut numbers and letters from places like Stahls. Depending on how many you need, it could also be an option to add in there.
thanks. It's actually a detailed design that I want to print black on White fabric and White on black fabric. I'm now thinking that silk screen printing using photo emulsion is the only real option.
The image will resemble this, along with some text. The image will be the tricky bit - and hence i still think screen printing will be the way forward.
This is the black on white version. White on black will be a negative of this. also, i would need at least 10-20 prints, and probably something that I can run off regularly. I don't have any kit as yet. No cutters, no screens, no printers. but as i'm starting out with this hobby, I will build up the kit over time. Screens are relatively cheap. Vinyl cutters may be a bit too pricy to start out with.
white text on to black fabric? 
