Link building via blog comments is a pretty controversial topic. If you asked most SEOs, they would probably rattle off the MOZ guide or say something about how most comments are nofollow and have no impact.
I’d say they are wrong.
Maybe not wrong in the general sense that going to Fiverr and just hiring someone to spam blogs with 1,000 comments for $5 won’t move the needle, but calling them wholly-uneffective is not telling the whole story, and I’ll tell you why (with examples).
I’ve seen two strategies work with blog comments, but I’d only recommend one as a sustainable practice that adds long-term value to your site.