A blog that is DoFollow contains no rel=nofollow tag in their links. Any links (whether in comment or post form) will be search engine friendly and can be crawled by Google and other search engine bots. This is a great function to help fellow bloggers raise their search engine ranking and Google Page Rank.
Remember that by default, Blogger and WordPress comments are set as rel=nofollow. If you are a blogger and would like to set your blog to dofollow, be sure to save your XML template and delete any nofollow tags in your layout.
If you would like to be able to see which links on websites are nofollow, you can download addons with that function. I use a Firefox addon called Link Checker to visualize which links to my site are dofollow (highlights green) and which are nofollow (highlights red).