Hello RwD,
If you have the 1.90bx version, then you can use a custom PHP function on the block using the 'onsection' parameter.
With previous versions, or with the same version, you can also use something like this:
{box.page_template;file={val};if '{val}'!='default'}
{box.page_content;htmlconv=look;protect=no;if '{box.page_template}'='default'}
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Because there is a nested TBS field, you have to be sure that in your query, the 'page_template' column comes before the 'page_content' column.
Your code the tbs_check can't work because tbs_check tags without sub-names are processed like fields and not like blocks.
It is not a good solution anyway because it first displays all contents (both page_template and page_content) and then deletes the bad ones. Which is not the fastest way.