Many months ago, when I first learned about dofollow blogs and the SEO benefit that commenting on them confers, I did quite a bit of searching for them. I amassed quite a substantial list and did comment on a few here and there. I then gave it up for quite a while.
I just returned to it recently, intending to write a few more comments. What surprised me was how many of the blogs had "died". And quite a few of them that were previously dofollow had been switched back to nofollow, presumably because the bloggers had become sick of all the spammy comments.
It just confirmed my view that while dofollow blogs are definitely worth commenting thoughtfully on if you find them, you can waste quite a bit of time and energy with this particular method of link building. I think that the best approach is simply to find blogs you like, return to them from time to time, and comment when you feel like it.