The simple answer is YES, it does help, but there are a lot of variable involved. A successful blog commenting campaign, with your keywords and URL attached in the header or footer of the comment can help your website immensely.
I have a reason to belive that links from blogs carry more value for search engines than link from websites. Even though I do not have concrete proof of it (because I was NEVER tracking it), blogs are favored because they are known for providing unique content. And everyone knows that search engines LOVE unique content.
But before you start commenting on random blogs, you have to make sure that you follow the following guidelines:
1. Make sure the blog is DoFollow. If it isnt, commenting on it is a sheer waste of time.
2. Try to comment on blogs related to your niche/industry. This would provide you with themed links (allegedly more linking power) and additional qualified traffic for your website.
3. Make sure you add value when you comment. No one will accept your comment if it is crap or if it is promoting weight loss or Viagra products blatantly.
4. Always look at previous comments on the blog and see if others were able to slip their precious keywords into them. If they were able to, chances are you can get away with it too.
5. Do not spam! It is counter-productive.
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