Mr. de Blasio, who has raised money for the Campaign for One New York, has argued that the activities of the nonprofits tied to him are aboveboard because they voluntarily disclose all of their contributions, despite not being required to do so. “It’s not dark money if it’s disclosed,” he said in February.

But the board said the nonprofit’s disclosures fell short of being “comprehensive” and fully accessible to the public.

On Wednesday, a spokesman for the mayor, Eric Phillips, said that Mr. de Blasio had been a “lifelong advocate” of campaign finance reform, though he did not touch on the specifics of the legislation the Campaign Finance Board and watchdogs have suggested.

“He looks forward to continuing the work to help get big money out of politics and to make sure everyone is playing by the same rules,” Mr. Phillips said.

The board’s actions were far more muted than its words.

Responding to a February complaint from Common Cause New York, a watchdog group, the board determined that because the nonprofit’s activities on behalf of the mayor had focused on his policy goals and took place more than three years before the election, there was not enough information to conclude that the organization had coordinated with the 2017 campaign.

The board also clarified rules governing nonprofit advocacy groups that are affiliated with politicians. If such groups coordinate with a politician’s campaign to promote the politician in the run-up to an election, anything they spend will be treated as a donation to the candidate and subject to normal campaign finance limits.

Run by former advisers to the mayor’s 2013 campaign, the Campaign for One New York functioned as a policy advocacy group, not a shadow re-election committee. But it checked a few of the other boxes the board outlined on Wednesday: Mr. de Blasio has helped raise funds for the group, and most of its spending has been on political consultants who routinely advise him.