Blog Commenting is as effective as it is (either do follow or no-follow) if you don't spam on blogs at all.
As we all know, blogs are not just created to put links and get backlinks. Blogs are created for contents and for readers to share factual information and knowledge as well as opinions.
When you comment on a blog, you have to really make sure you have read whole content of the blog and what it talks about. Blog can be a good way of interacting with other people who had read blog - either by opposing it or agreeing with the post.
So where is the benefit of it? Adding a link to your comment is actually just an EXTRA benefit. The real benefits of commenting to a blog that you have read is that:
1. You gain knowledge or probably corrected the blog posts
2. Your comment is actually considered as a content, and if you post a quality comment, more likely you are also posting a quality content.
3. You have a higher chance to let your link stay there and provide a strong backlink for your website, especially when you are trying to link on a .edu or .gov site.
4. You can avoid being seen as just a spammer on the blog.
5. As you gain other people commenting on the blog, the chance of them visiting your website, and then probably sharing it to other people that they know could be another benefit of what you can get from it (which is traffic)
Of course, you only comment on blogs that are related to your portal, and with at least a high PR
- does not necessarily also that it should be a do-follow at all.