Commenting on the blog is close to worthless, for several reasons.
1. Most blogging software has atrocious handling of comments. Even such a basic feature as threading within the comment stream is often missing.
2. There often is not any kind of way to vote on comments so as to make it easy for people to find the good comments.
3. Bloggers are often one-hit wonders. They write one good blog post that makes it to sites like HN and Reddit, and then fade back into their normal obscurity. This makes it much less likely for a community of regular commentators to form around any particular blog. On sites like HN, one starts to recognize the frequent commentators, and see what they think on a variety of different comments.
There are often times where the comments here or on Reddit are sufficiently informative that I don't even get around to clicking through to the original article.