How much effecting blog comments ?

I have done blog commenting before and it really does work. I have found that it can be quite helpful.

I've wrote many articles about this, but in my opinion and from my studies, yes, having blog comments on a blog post does help increase its rankings.

It shows Google your content is interesting and engaging.
It also adds value to your blog post if those comments are indexable (e.g. Disqus comments plugin).

Just don't try to go all black hat with it and add a ton of crappy comments to a post to increase your rankings. Google knows the difference, and time, quality, and variation all play a factor.

Hope this helps.

Of course it does work, there is a reason that people has been doing it for a long time. You should be going for high quality comments which don't look automated if you want to have success with it though.