I would love your tips on how to spot/avoid blackhat SEO providers. Also what are the most common blackhat tactics?
Keyword Stuffing: trying to stuff as many keywords as possible in the meta tags of a document, and Hidden Text: text that is hidden from the visitors view but is readable by a search engine bot, are very common in the world of BlackHat SEO.
Doorway pages: pages that are created specifically for search engine bots; these pages are also choked full of keywords and phrases to help rankings, and Page Cloaking (commonly used with Doorway Pages): this occurs when the webmaster creates two separate versions of his/her page, one for search engines and one for the normal visitors, are some other tactics that are used.
Do some research online about SEO provider and check if anybody has complaint about them.
Review SEO provider OR SEO Company's employees profile on Linkedin/Google maps. Check the SEO provider website, if he/she is doing any black hat techniques.
Blackhat SEO is unethical, treading near the criminal in certain instances, and a topic we generally don't like to explore at SitePoint. We're family-friendly in more ways than one. Besides, if you do a forum search, I estimate you'll find approximately one quintillion posts on the subject. Thread closed.
However, here's a Google search to get you started on the subject.