It is a good idea to be signed up for a Google account (and others). When you sign, be sure to include an image for an avatar. That way your comments will be less likely to get lost and when they do appear, they'll do you more good because your logo/avatar will help with your branding.
Using blogs (your own and others') is something you'll learn more about in my Frugal Book Promoter: http://budurl.com/FrugalBkPromo, along with hundreds of other ways to promote your book.
My husband has a new book out and by commenting constructively on suitable blogs and forums, he's been selling about 20 books a day. As a newbie! Of course, it is nonfiction and much needed by his target audience, but it still goes to show that comments (done right!) work. Stephanie Meyers (the author of the Twilight series of books and movies) writes fiction and she used comments as a major part of her personal marketing campaign ("personal" meaning what she did apart from what her publisher did). In fact, the way she used blogs to market was covered in the business section of the LA Times! Obviously there is more than one way that blogs can benefit your marketing campaign.
Hope this helps!
Carolyn Howard-Johnson, author of This Is the Place; Harkening: A Collection of Stories Remembered; Tracings, a chapbook of poetry; and how to books for writers including the award-winning second edition of, The Frugal Book Promoter: How to get nearly free publicity on your own or by partnering with your publisher; The Frugal Editor: Put Your Best Book Forward to Avoid Humiliation and Ensure Success; and Great Little Last Minute Editing Tips for Writers . The Great First Impression Book Proposal is her newest booklet for writers. She has three FRUGAL books for retailers including A Retailer’s Guide to Frugal In-Store Promotions: How To Increase Profits and Spit in the Eyes of Economic Downturns with Thrifty Events and Sales Techniques. Some of her other blogs are TheNewBookReview.blogspot.com, a blog where authors can recycle their favorite reviews. She also blogs at all things editing, grammar, formatting and more at The Frugal, Smart and Tuned-In Editor .