Blog tours don't always cause a big spike in sales, but they do fuel word of mouth for your book and build name recognition. Each time you appear in front of your target audience, it's a plus. When a blog appearance goes particularly well, don't let it end there. Offer to write a monthly guest column for the blog in exchange for a link to your site and a permanent buy-the-book affiliate link.
Map out your blog tour strategy a few months before your publication date. But don't make the mistake of scheduling a blog tour or any other publicity before your book is available for sale. Blog tours spark impulse sales, so make sure anyone who wants the book is able to buy it.