Seek legal consultation.

With that being said, if they own the right to publicity or copyrights now or represent the artist, it's going to be tough to fight. If it was a personal blog they may be able to go after you personally. But if they are a legitimate rights holder you will spend multiples more trying to fight it on the basis of not knowing about copyrights or not having made money from it.

You should request documentation to the matter and speak with an attorney.