What are the risks of hiring a black hat practitioner?

Disclosure. Embarrassment. Content being removed (or restored) to articles. Less rigorously sourced material is more vulnerable to rewrites. Less rigorously written articles are more likely to attract the negative attention of editors. Even if not caught in the short term, “black hat” articles generally aren’t as good because of all the corners that are cut. So you’ll end up with a lower quality article.