Filters blocks to automatically add aria-label to links that open in a new tab.

accessible-external-links.php

<?php
/**
 * Filters blocks to automatically add aria-label to links that open in a new tab.
 * 
 * @param string $block_content The block content.
 * @param array  $block         The full block, including name and attributes.
 */
function who_accessible_external_links( $block_content, $block ) {
	$blocklist = array(
		'core/button',
		'core/paragraph',
		'core/list',
		'core/heading',
		'core/quote',
		'core/pullquote',
		'core/table',
		'core/file',
		'core/image',
		'core/gallery',
	);
	/**
	 * Filters blocks that have to be processed.
	 * 
	 * @param array $blocklist An array of block names.
	 */
	$blocklist = apply_filters( 'who_accessible_external_links_blocklist', $blocklist );
	if ( in_array( $block['blockName'], $blocklist ) ) {
		$processor = new WP_HTML_Tag_Processor( $block_content );
		while ( $processor->next_tag( 'a' ) ) {
			if ( '_blank' === $processor->get_attribute( 'target' ) ) {
				$processor->set_attribute( 'aria-label', esc_attr__( 'opens in a new tab', 'accessible-external-links' ) );
				$processor->set_attribute( 'rel', 'external' );
			}
		}		
		return $processor->get_updated_html();
	}
	return $block_content;
}
add_filter( 'render_block', 'who_accessible_external_links', 10, 2 );