This is what I learned (and yes I fallen in the same trap before), and I'm assuming you'll use a common template for your sites.
If you use a subdomain, it will be treated as a separate website.
If you're making a blog via a subdomain, then at least have other pages attached to that subdomain as well and accessible so that your blog does not look like a "doorway" page.
For example, you can have a blog page for the homepage of the subdomain, and then have a link to information about who writes the blogs, and another link to an option to talk to someone about the blogs and maybe another link to information as to why the blogs are started, etc. What you want to avoid is having your template (such as a common website footer) containing common links on the blog page with no other links about the blog because the common links will be outside of the subdomain.
If all you are able to do is one or two pages about blogs, then you're better off having just a url from the same domain as the rest of the site.
You can then verify your setup with powermapper tools at:
Then put the URL of your blog in the box and check for errors and if there's one about avoiding doorway or cookie cutter approaches with regards to the URL, then go on the safe side and make domain of the blog URL the same as the rest of your site. For example:
http://www.example.com/ecommerce <-- for the ecommerce section
http://www.example.com/blog <-- for the blog