Normally, when you would like to have a blog, you will need to create another sub site or site collection.
I found nice trick from Steve Mann blog to create blog to your existing site.
It’s very straight forward by executing below powershell in SharePoint 2013 Management Console:
Enable-SPFeature -identity "BlogContent" -url <http://site url>
Once you’ve run the command, you will have “Post” included in your SharePoint 2013 Apps list.