render-reusable-block.php

<?php
/**
 * Renders content before the blog starts. Using a reusable gutenberg block.
 */
function generate_blog_intro() {
	/**
	 * Make sure we are on the posts page.
	 */
	if ( is_home() ) {
		/**
		 * Get the raw post object from reusable blocks.
		 */
		$block_post = get_post( 999 );

		/**
		 * Only render if the post exists.
		 */
		if ( ! is_wp_error( $block_post ) ) {
			/**
			 * Re-renders all the blocks in the content of the post.
			 */
			echo do_blocks( $block_post->post_content );
		}
	}
}
add_action( 'generate_before_main_content', 'generate_blog_intro' );