Here’s a useful tip for those who believe that their previous employer may be blacklisting them or providing negative references. There are companies that will conduct reference checks on your behalf by posing as your potential employer. They will call your previous employers under the guise of checking your references for a new position and report back to you.
This can be a useful service when it comes time to find a new job. There may not be too much you can do if your employer is deliberately giving you a negative reference, but at the very least, you can avoid listing that employer as a specific reference. Although in most cases you will have no legal recourse, if your employer is slandering you (i.e. making up falsehoods), then you may want to consider consulting with an attorney to explore your legal options.