Negatives developed as slides (reversals).

Entered in error. I thought I had a problem, instead I had a brain fart.

Where you by any chance holding the negatives up to the light to inspect them when you noticed this. I've found that if I hold a film at a certain angle with a bright light source almost directly behind the film you can actually see see a positive image. I've meant to ask about this before but never got around to it.

This only happens at a particular angle with the light behind.

If you've really got slides I'm stumped. B&W reversal process has several steps including bleaching with nasty acid.

i'll bet you were using a black light so blacks turned white and the whites turned black.