Not sure how your slides came into your iPad with black backgrounds - they should have retained their original settings. Once in iPad, it is more difficult to change the background as the iOS version does not offer a way to change themes on the fly like you can in Keynote '09 for OSX.
So short of re-synching your Keynote presentation from your Mac and hope that that fixes the background problem, you could create white color shapes and place on the the troubled slides and Styles (brush) > Arrange > Move to Back so that the white is the bottom layer below your text.
Another fix is to copy each slides objects (select all slide objects - text, photos, shapes, etc. ) and close your presentation and create a new presentation (Tap the "+" Button in Upper Left in Document View) and select a different theme/template) and Paste the slide contents on a blank slide, repeat for each slide until you have a working copy.
Tedious, but if you can't import your Keynote from your Mac correctly, there are few options.