The layout for Category Blog is actually is defined in this file :
If you open the file, you will see this file will call these child layouts :
So, basically you can change the layout for blog layout by modifying those files above.
But, it is NOT recommended to modify those files directly because any change you made on those files will be overwritten when a Joomla update is installed in the future.
So, how to deal with it?
Joomla has a template override features that allows you to override visual output of any extension you have. You can read the fully explanation here but in your case above, you just need to copy those files above to :
and modify it.
If the folder 'html' doesn't exist on your template, simply create it.