By the way, blogger blog is nofollow blog in default. Nofollow is not important for SEO. When you make your blog dofollow, you might get lots of comment as well as spam comments. So you should able to face with spamming because webmasters search dofollow blogs and treat them for backlinks. However, you get also lot of original comment if your blog is unique and good. In order to make your blog dofollow comment blog, just follow the given steps:-
Step 1: Lets enter into the blogger dashboard.
Step 2: And then just click on template and edit html.
Step 3. Now let just find the given code. Press CTRL and F then find this code
Step 4: Just replace the code nofollow by dofollow otherwise just delete the above code.<a expr:href='data:comment.authorUrl' rel='nofollow'><data:comment.author/></a>
Step 5: Now click on Save Template.