Authenticity is very ill-defined (just ask any hipster). What makes an authentic company blog in your mind? Does it have to written by an employee of the company? What if it the company is entirely outsourced (except for the owners, obviously)? Does that mean the owners must write it? What if the owners aren't any good at writing? Can they not hire someone to do this job for them? Let's say we're talking about a software company, and all development is outsourced. Is the product now inauthentic? If not, what's the difference between a blog and the product?
Interesting and insightful blog posts are usually written by someone with intimate knowledge of some part of the business, be it an outsourced developer, a founder or even a marketing intern. The proximity matters. That knowledge is not going to be transferred over a five input form.
One type of authenticity speaks to origin an item. Whether it be a painting, writing, or any other work, when a work's origin is in doubt, it can be said to be lacking authenticity. The inauthentic part of ghostwriting is that the name of the author on the content is not the person who created the content.