I hate to point to another moz article, but this one is a bit more recent and talks about why subfolders are generally preferable to subdomains - http://www.seomoz.org/blog/understanding-root-domains-subdomains-vs-subfolders-microsites.
The short answer is: depends, but with modern search engines, it probably doesn't make much difference. Search Engine Journal has a fairly good article on this topic.
I would argue that it should follow the convention of the rest of the site. So, for instance, if your site has http://www.mysite.com/learn/, http://www.mysite.com/explore/ and http://www.mysite.com/meta as major portions, use http://www.mysite.com/blog/. If it has http://learn.mysite.com/ and so on, use http://blog.mysite.com/.
I personally follow the one-subdomain-per-major-product-group philosophy, and the blog covers all product groups, so I give the blog its own subdomain. YMMV (Your Mileage May Vary).