Yes. This is legal.
The only possible liability for a truthful and accurate disclosure of fact is a defamation action (in the absence of a privacy clause in the contract) and this is truthful so it would not violate anyone's legal rights. Credit reporting agencies routinely collect such information and court actions to collect unpaid debts are also a matter of public record. Credit reporting agencies in this business also have some additional obligations (such as the obligation to remove an entry after a period of time and an obligation to present rebuttal statements from the person affected).
But, you should understand that merely publicly sharing truthful information about a factual matter is not really what a "blacklist" means. Normally, a blacklist includes an implied understanding that certain actions will be taken as a result of placement on the list rather than merely sharing information for what it is worth.
An example of a law prohibiting a true blacklist from Colorado is the following:
§ 8-2-110. Unlawful to publish blacklist
No corporation, company, or individual shall blacklist, or publish, or
cause to be blacklisted or published any employee, mechanic, or
laborer discharged by such corporation, company, or individual, with
the intent and for the purpose of preventing such employee, mechanic,
or laborer from engaging in or securing similar or other employment
from any other corporation, company, or individual.
Incidentally, I'm not convinced that the statute would be constitutional if enforced under modern First Amendment jurisprudence, although one U.S. District court case from 1971 did uphold its validity in the face of a somewhat different kind of challenge. Resident Participation, Inc. v. Love, 322 F. Supp. 1100 (D. Colo. 1971).