Black hat SEO is not legal and if someone use this he/she must avoid some factors as:
1. Unrelated Keywords
2. Keyword Stacking and Keyword Stuffing
3. Tiny Text, Hidden Text, and Hidden Links
4. Cloaking
5. Doorway Pages or Gateway Pages
6. Bait-and-Switch or Page Swapping
7. Duplicate Content or Mirror Site
8. Spam Blogs or Splogs
9. Blog Spam or Comment Spam
10. Trackback Spam
11. Spam Ping or Sping or PIngback Spam
12. Referrer Spam
13. Link Farms
14. Cybersquatting or Domain Squatting
15. Typosquatting or URL Hijacking
16. Social Networking Spam
17. Cookie Stuffing or Cookie Dropping
Therefore, i only to say that all these above factors will impact bad impression on your site.

Black hat seo is illegal only for search engines, and not matter for other online communites.