Different themes on one site

You probably are over complicating things for yourself. You can use the template_include filter for that

add_filter( 'template_include', 'wpse_206394_template', 99 );

function wpse_206394_template( $template ) {

    if ( template should be blabla  ) {
        $new_template = locate_template( array( str_replace('.php','-blabla.php',$template) ) );
        if ( '' != $new_template ) {
            return $new_template ;
        }
    }

    return $template;
}

but most likely you can get the same effect with proper use of the template hierarchy.