I don't know about other states but in Va. a past employer can't not legally give a bad referance.
But, as the saying goes, "it's not what you say but how you say it".
A question future employer can ask of a future employer is "Is this person available for re-hire?". An anwser would be a bad thing.
If you would give me an application and did not have past employement or missing dates, you better have an explanation or it will go in the trash.