I think it's not that really important to know if the blog is a dofollow or nofollow. What matters is the traffic of real visitors. Besides most blogs online are nofollow because they have a hard time dealing with spammers years back. You want to get on top to SERPs so that your site will get easily noticed by visitors. But you can still get visitors on popular blogs.
If you post your comment on blogs that are relevant to your niche, you are more likely to redirect your targeted customers (people who are more likely to buy your product).