How to bring up to a future postdoc employer, the point of not asking for recommendation for a specific postdoc employer?
It would be unprofessional to describe your previous employer in anything but positive terms when you are a job applicant.  If you cannot describe them positively, do not describe them at all.  You are showing the future employer how you will describe them in the future.
You could try suggesting an alternate reference, without saying why.
Ask yourself, why would the potential employer make an unusual request for a specific reference if this was not a critical factor in their hiring decision?  Your efforts might be better directed towards a different employer.