[single.php] Single Post Subcategories?

Currently I’m using a single post template for a certain category, e.g. Blog. All the post directly in category ‘Blog’ are using the layout in single-blog.php

However I defined serveral subcategories under ‘Blog’. The template isn’t working for subcategories, I have about seven of ’em. How will I apply the parent single-blog.php template to all subcategories filled under ‘blog’?

Creating seven PHP files referring to single-blog.php would be a solution but there must be a better way!