One of the oldest forms of link-building is blog commenting. On one hand, it has never been the best approach since all-too-often links that visible in blog comments are no-follow anyway and the sites that have dofollow plugins are often flooded with links to poor sites, potentially putting your links in a bad neighborhood. On the other hand, it’s not something that Google will penalize the same way the penalize forum signature spam as long as you’re not doing it to manipulate the system.
That’s the key, really. Don’t try to manipulate. Commenting on blogs can be an excellent way to get exposure to your topic. For dealers, it can help to bring traffic to your blog or resources on your website, particularly if you’re doing it in a way that brings value to those reading the comment. However, this is not something that should be part of your SEO strategy. Again, think value rather than inbound links when leaving comments on blogs.
The bottom line is this: the real juice when it comes to linking through comments is in the opportunity to get links through value. Here’s an example… let’s say you have a post on your blog about changing connecting the new iPhone 5S to a particular vehicle. As you read through blogs about both automotive iPhone connectivity and posts about the vehicle type itself, you may be able to answer questions or offer your post as a resource in the comments. Those who read the article or even the original article writer may find enough value in your post to link to it as a result. Worse case scenario is that you have a comment link to your site that may or may not bring traffic and may or may not bring some link juice. The best case scenario is that you expose your blog as a resource that others are willing to link to or share on social media.
Value, value, value. Don’t spam.
Here’s a brief answer to the question from Google’s Matt Cutts: