I am often asked the question . . . "They can't do that to me, can they?" The answer is, "it depends. I would need much more information about your type of employment, circumstances of your termination/discontinuance of employment and a little insight as to the motives your former employer may have for taking this extreme and troublesome position. The only way to fully analyze your situation is through a no-charge office consultation. You can make an appointment by contacting me at firstname.lastname@example.org.
Your legal rights depend on what your ex employer is saying about you and why.
He cannot "blacklist" you by make misrepresentations about you. Also, if he is attempting to interfere with your prospective employment relationships for an unlawful reason, that would be actionable, as well. However, if he simply refuses to have a business relationship with a company that employs you because you left him, I cannot think of a legal claim you might have.
Perhaps, the collective wisdom of my colleagues can come up with an idea. (Constructive non-compete against public policy, anyone?)
They say you get what you pay for, and this response is free, so take it for what it is worth. This is my opinion based on very limited information. My opinion should not be taken as legal advice. For true advice, we would require a confidential consultation where I would ask you questions and get your complete story. This is a public forum, so remember, nothing here is confidential. Nor am I your attorney. I do not know who you are and you have not hired me to provide any legal service. To do so would require us to meet and sign written retainer agreement. My responses are intended for general information only.
As Mr. Murray points out, additional facts would be helpful to an analysis, but might be more appropriate to a private attorney visit rather than in a public forum such as Avvo where no attorney client relationship exists and therefore no attorney client privilege. A preliminary analysis is that this might be interference with contract or interference with prospective business advantage because of the affirmative acts of your former employer.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
I do concur with my colleagues that additional information is necessary to determine the actions, if any, that could be taken in your situation. I like tortious interference with prospective relationship especially given the malicious motive involved. I would also look at possible defamation if your former employer publishes defamatory statements about your future employer. But it all depends on what your former boss does to "blackmail" your future boss.