Once upon a time, SEOs felt rather pleased with themselves when they came across a 'dofollow' blog which would allow them to post a link or two within a blog comment, in order to help their websites rank a little better. Should the comment be approved, the links to the pages they quoted would be crawled by the search engines, and they would, in time, benefit from a small increase in authority and PageRank.
So what has changed?
As search engines continue to evolve and as their algorithms are able to consider more and more complex factors, commenting on blogs has become somewhat of a dated - and in some instances, a frowned upon means of SEO.
The fact is, if you go about spamming blogs with comments, then it's likely that you will be frowned upon. Webmasters will quickly become irritated by comments which are full of links and which don't necessarily relate to any point made within the blog itself. Often, those comments are deleted and are never made live in any case - wasting both your time, and theirs.
Should I not post comments on blogs anymore?
But, there's nothing wrong with posting links within blog comments - providing you do it in the right way. The key is to only comment on articles which you feel you are able to constructively contribute towards, and certainly only post a link if you feel other readers would benefit from information on your own site. If your site is off-subject, and you don't say anything more than just 'good article', you aren't contributing much opinion and value to the blog or the wider Internet.
Focus on the relationships you can build through blog commenting
Blog commenting is a good way of building up relationships with other people interested in similar things to you, and you should look at it from this perspective. You won't get much recognition if you don't contribute your thoughts, but instead, spam blogs with worthless comments. If you comment on blogs, make sure you do so for the right reasons.