if i have a website in a certain niche and i go to some blog on that niche and comment there (a serious comment) and also put a link to my site, does google read it as a backlink? does it have any affect?
Yes, it is good for getting a good back link from blog of same niche but too many such links may be harmful for your site. Try other sites too except blogs.
Blog commenting on authority blogs help to generate good traffic to your website. It helps to build your traffic and also make your website popular in search engines.
Sometimes, the blog comment section can be extremely interesting and informational too. True comment marketing is about sharing meaningful opinions make connections and earn exposure.