functions.php

<?php

add_action('init', 'remove_parent_theme_shortcodes');

function remove_parent_theme_shortcodes() {
remove_shortcode( 'blog_posts' );
add_shortcode( 'blog_posts', 'blog_posts_custom' );
}

function blog_posts_custom($atts) {
	echo 'test'
}

add_shortcode("blog_posts", "blog_posts_custom");