I want to make major changes to the layout of the Liferay blog portlet. Is it possible to define my own template somehow? I want to be able to change the markup. Kind of the same way you can make templates for Web content.
Unfortunately the blogs portlet doesn't have the same capability as WebContent. However, depending on your setup, you might be able to "mimic" a blog with WebContent: Categorize your content according to your requirements, publish on AssetPublisher and stick a label "Blog" on to it.
It depends a bit on the number of blog authors and the location of the blogs you have: After all, you'll have to enable them to author/edit webcontent in one site. If that's not a problem, use this. As you might use a defined template, you might also be able to automate a bit of extra work when you provide a custom portlet for article input - but that would be the advanced usecase.