There are several types of fraud. The legal definition is (1) a knowing misrepresentation of the truth or concealment of a material fact to induce another to act to his or her detriment, (2) a misrepresentation made recklessly without belief in its truth to induce another person to act, (3) a tort arising from a knowing misrepresentation, concealment of a material fact, or a reckless misrepresentation made to induce another to act to his or her detriment, (4) unconscionable dealing.
Civil tax fraud is an intentional evasion of taxes (for example, not paying state sales tax). Criminal tax fraud is a willful evasion of taxes (for example, not reporting taxable income on a tax return).
Unless the blogger knows for a fact that sales tax is not being paid or income is not being reported by the artisans, her use of the word "fraud" to describe an artisan without a sales tax permit could be defamatory. She is implying tax evasion based only on her knowledge of permits applied for and obtained.
Fraud also involves misrepresenting or concealing a fact that will induce someone to act to their detriment. The fact that someone does or does not have a sales tax permit is not exactly an inducement to purchase a product. This fact is not material to the purchase, and purchasing a product from a seller who does not have a tax permit is not detrimental to the purchaser.
So, you and your artisan friends could potentially have a defamation action worth pursuing, if what the blogger is saying is false. If an artisan can be shown to have not paid taxes, however, or if the blogger can in any way prove what she has posted, those artisans may want to rethink bringing an action against the blogger.
One or more of you could go to court with evidence of the defamatory content on the blog and request that the judge issue an order to Google to have the material removed and/or access to the blog disabled. In addition, you could have the court subpoena Google for the identity of the blogger (if it is not already known), so that you can proceed with a defamation action against this identified person.
Often courts will not issue orders or subpoenas unless or until a lawsuit is filed (an anonymous poster would be "John Doe" for the purposes of the suit), so you may want to consult with an attorney prior to going to court. The attorney can review the blog postings and the information from each of the defamed artisans and give all of you a better idea of what can be expected from a suit (jurisdiction-wise, defense-wise, win-wise, expense-wise).
Defamation actions are expensive, especially when more than one state is involved. Internet actions can be especially difficult. Serving Google, alone, will be an undertaking that can get costly unless one of the artisans and the attorney who is hired reside in Santa Clara County, California (home of Google and the place where court orders and subpoenas should be served).
Good luck.