I remember back in 1976 Toronto came within a hairsbreadth of getting the San Fran Giants. The deal was signed and ready to go and the new mayor Moscone got a court to halt the deal until another buyer could be found. Bob Lurie eventually bought the team. So a mayor stopping a sale has happened before. The next year an expansion team landed in Toronto instead.
The economics problem still exists today in the bay area though....two teams within a twenty-mile radius marginally works.
In this case...respect for de Blasio is a joke, and baseball leadership knows that.