Has your blog got any authority itself? Google will read the links, but your blog won't pass any juice if it has no juice onto itself.
Blog links are no different than any other links to Google. Most blogging platforms and software are set to not pass juice by nofollowing the link though, specifically to stop the comments being spammed and abused by every black and Grey hat SEO monkey under the sun.
The keywords used in comments are considered for the page where they appear - even though links may be nofollowed. It can even be frustrating to find a result where the query term is only used tangentially in a blog comment and the page really isn't about that topic in any way.
However, I would suggest keeping your comments natural and informative. Blogs are, of course, a lot more than a technical SEO tool. Your regular readers deserve real value.
@tedstar--yes it's frustrating...just found a page without the query term, but it was in the comments of that page...and yes the users deserve real value...and can Google identify me while writing a comment on my own blog (blogspot)...I write quality comments as like my contents...haha >=]