While commenting on relevant blogs can be a good way to build relationships with blog owners, it is a horrible way to improve your SEO. From a pure traffic standpoint, very few people tend to click on your comment link.
From a linkbuilding perspective, you are getting a minuscule to zero link juice going back to your site. Since 99% of blogs automatically no-follow comment links, they send zero link juice to your site. No-follow communicates to Google that the site owner does not endorse the link and therefore it does not count as a vote for your site.
If you can find blogs that take no-follow off their comment links, it can pass a small amount of value. But typically there are tons of comments and your link juice gets divided between the other links on the page.
Note that it doesn’t say don’t comment. Comments are a valid way to create relationships with bloggers.
There also is marketing value to comments. Think of it this way. You are using someone else’s platform to expose yourself to another audience. Offer something of value and you may gain some fans and grow the audience to your blog.