Blog commenting is a relatively easy way to build links. As with all link-building it takes time to build enough links to make a difference to the performance of your site in SERPs. Like forum posting, blog commenting also has a timeout factor. As new posts are made on the blog, the post you commented will probably fall off the blog home page and the link juice being passed to your site will tend to fall away.
There is an opinion too that the search engines can easily spot a WordPress site and tend to devalue any links in comments. The idea behind this is that a link in a comment on a blog is a vote/ link for yourself and the links that really matter in terms of SEO are the ones you get naturally from other site owners.
However, building links is essential to SEO so my feeling is: If you get a chance to make a link, take it! Here are some tips for blog commenting.
Comment on blog posts that have PR
The whole idea behind building links is to get link juice to your site. If a post has PR ( > 1 ) all the better.
Comment on blog posts that don’t have PR
If a blog post doesn’t have PR today, who’s to say it won’t tomorrow. If you measure the PR of a post on a blog using the Google toolbar, it’s often the posts that are 2 or 3 months old that have PR. This is simply because the posts were created before the last toolbar PR export. Even though the newer posts have PR N/A or PR 0, really they will have PR – it’s just that Google isn’t showing it in the toolbar yet. There’s an extra advantage in commenting on the newest posts in a blog – most people blindly comment on the posts that show a non-0 PR. If you’re lucky you’ll get a couple of months with a comment on a post that hasn’t got 100 comments from other people.
Make sure a blog is dofollow
If you want to get link juice, making sure your comment link won’t have the rel=nofollow attribute is essential.
Don’t worry too much about dofollow/ nofollow
Even if a blog is nofollow, you might still get traffic from a link on a good, well-written comment. Some of my best referring links are nofollow.
Comment on blogs in the same theme
For the best effect on SERPs performance comment on blogposts on the same topic as your site.
Don’t worry too much about the theme
Link juice is link juice. If the theme is off topic, don’t worry – the extra link juice will still bring some benefit.
These tips may seem contradictory but there are sound reasons behind each of them. Remember – see a chance of a link, take it!