On Joomla 3,this simple override adds the subcategory title above the relevant articles in category blogs. The result looks like this:
Subcat title A
Article 1
Article 2
Article 3
…
Subcat title B
Article 1
Article 2
Article 3
…
Subcat title C
Article 1
Article 2
Article 3
…
For an example see [http://svictor.net/anthropology/publications] (Books, Edited volumes, Articles etc. are all subcategories of Publications). Stock Joomla can sort the articles by subcategory, but doesn’t output the subcategory title above the relevant set of articles.
Place the file in /templates/your-template/html/com_content/category/
The override is only applied on pages with the "subcats" page class. To set the page class to "subcats" head to the options of the relevant menu item. There is a short snippet at the begining of the file to check whether the override is active or not (commented out by default).
The article set of each subcategory is wrapped in a div of class "accordion". As the name suggests, this can be used in conjunction with an accordion effect plugin.
The templates on which I tested this are very close to stock Joomla. Most of them are just html and css on top of the core files. I don’t use any additional framework. My override may not work if you use more sophisticated template systems, especially if they already have their own overrides for blog.php.
It is a very simple override. My additions to the stock file are indicated by comments starting with "V.".