You may offend a few people who felt you were trying to trick them but it's not Black Hat. In the eyes of google... Black Hat is trying to game search engine results. So even buying backlinks is Black Hat attempts at trying to manlipulate your sites ranking with google. About 10 years ago I was selling a ebook on a physical CD via ebay. So I loaded the CD up with several more PDF ebooks and password protected them. Then included a sales letter thanking them for their purchase and letting them know that if they liked the ebook they paid for... there was several more on the CD that they now held in their hands. The letter talked about each of the ebooks and was upfront about the extra ebooks being password protected along with instructions on how to pay to get the password and gain access to the product that they already held in their hands. This creates curiosity... the only way for them to satisfy their curiosity was to either stop thinking about it or buy the password. Worked like a charm!