My opinion on blog comments?
a) Creating lots of blog comments on irrelevant sites with generic 'thanks for the post' comments just looks spammy. As others on here said, it doesn't look good and can raise a flag.
b) the approval rate for blog comments is really low, most are no-follow (pagerank doesn't flow) and it is hard and time consuming to find blogs with a good pagerank . You're much better spending the time it takes to make a blog comment working on content that could potentially get thousands of natural links.
c) Lastly, there are some rare exceptions when you might get a link from a blog comment that is worthwhile. Engage in blogs that you feel fit your niche to build reputability and you may find yourself gaining from it - but don't use them as a method of link building.