I don't think the potential for success is so huge. Many blogs are low-quality, many are even running on subdomains like wordpress or blogger. Blogs generally are personal sites, not for profit business sites. So most bloggers are not going to pay a lot for a domain name. On the other hand many companies have a blog. Some will probably buy their corporate name in .blog as an addition to their current site. But then it is somewhat redundant. I don't like the idea of fragmenting your online presence - purpose-specific extensions such as .mobi or .tel failed for good reasons.
And of course the most desirable keywords will be out of reach for domainers and end users alike.