To modify the internal blog template files go to this directory "modules/addons/blog/templates/default"

To modify the external blog template files go to this directory "BLOG_ROOT/templates/default"

To create your own template follow this instructions

1. Duplicate the default template folder and rename the folder with a new name
2. Modify the tpl files as you like
3. To select the template go to the blog settings then seclect the template from the General Tab > Template