Yeah . you may be right. If you are getting link from a site whose only purpose is to send out links then it will harm you.Because Google may think that your site is manipulating external links. Another good thing is that if you comment on a blog, the blog owner will surely check out your site and thus you can develop a good friend circle online. "blind commenting" will definitely harm you as you said.
I think you are right blog commenting are old method and now google is degrading this technique and many blogs do this spam and it goes to black hat technique.
You will find that some good do follow blogs will become no follow in no time due to huge amount of spammers online. If I have that much time, I will dedicate that time for content development of my website .
Thanks for the replies guys. You are welcome to add in more techniques that have been effective for you.