Its depend on policy of Blog Moderator or Admin so you should select those blog which approved already some other comments with backlinks.

Yes I agree with Varshy, It is depend on Blog's owner. You can try Instant approval blog comments blog for this.

I agree with varshyl, I wouldn't suggest to select auto approve blog but some blog which accept keywords in place of name, if Moderator will that comment is real then will be approved

In my opinion you have to give a comment that should be genuine and topic based and don't give any link in your comment. Besides this there is an option for url validation, so you have to use your site's url only there.

I used to use blog comment method to get results and i can get more results with it. But just the need if to provide quality and unique content in your post ad write some meaningful that people like to click on your link and like to see that what kind of information there is that they can be found meaningful.

Getting a link from a reputable blog is quite hard in form of blog comment according to me you should try Guest blogging, its the effective way of getting do follow links from decent page rank website. Basically blog commenting is entirely depends on property of blog and its Admin.