Everyone know that Blog Commenting is the good way of getting traffic but I want to know that Does Blog Commenting really effects the SERP and Alexa ranking of the website.

If your comment have quality and blog theme match with your site category than it would be great. For getting maximum of your comment, Try to get these links from Do Follow blogs.

As per me dofollow comments really helps u to increase your ranking in positive way and effect SERP and PR.

Yes, if you are hyperlinking keyword text.  if the comment does not link back to your website then NO it will not help your rankings at all.

Blog Commenting is one of the part of the SEO Off-Page method and it is the best Link building method. Through blog comments you can get more and more traffic to your website and as a result you can get the backlinks and your site will get the promotion on the search engine.

Do follow comments can help your ranking go up, and you shouldn't stop commenting on do follow blogs.