Adding a class to external links in content blocks.

functions.php

<?php

namespace MyPlugin;

// Adds a class to external links in content blocks.
add_filter( 'render_block', __NAMESPACE__ . '\\add_class_external_links', 10, 2 );

/**
 * Adds a CSS class to external links in content blocks.
 *
 * @link  https://developer.wordpress.org/reference/hooks/render_block/
 * @param $block_content  string  The block content about to be appended.
 * @param $block  array  The full block, including name and attributes.
 * @return string
 */
function add_class_external_links( $block_content, $block ) {

		// List of content blocks.
		$blocks = [
			'core/heading',
			'core/paragraph',
			'core/quote',
			'core/pullquote',
			'core/preformatted',
			'core/list',
			'core/table',
		];

		// Cancel early if necessary.
		if ( ! in_array( $block['blockName'], $blocks ) ) return $block_content;

	    	// Get this domain.
	    	$url    = parse_url( site_url() );
	   	$domain = $url['host'];

		// Let's start the content parser.
		$dom = new \DOMDocument('5.0', 'utf-8');

		// Enable libxml errors.
		$internal_errors = libxml_use_internal_errors( true );

		// Need to force conversion to specific encoding standard to prevent errors.
		$dom->loadHTML( mb_convert_encoding( $block_content, 'HTML-ENTITIES', 'UTF-8' ) );
		libxml_use_internal_errors( $internal_errors );
		$xpath = new \DOMXPath( $dom );

		// Find all links that does not include this site's domain.
		$links = $xpath->query( "//a[not(contains(@href, '$domain'))]" );

		// If there are links to an external domain.
		if ( ! is_null( $xpath->query( "//a[not(contains(@href, $domain))]" ) ) ) {

			// Add the class to the link.
			foreach( $links as $link ) {
			   $link->setAttribute( 'class','epc-external-link' );
			}

			// Save the block content.
			$block_content = $dom->saveHTML($dom->documentElement);
		}

    // Return the original block content.
    return $block_content;
}

/**
 *
 * Style external links
 *
 */
.epc-external-link::after {
	padding: 0px;
	content: '';
	width: 16px;
	height: 16px;
	display: inline-block;
	mask-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24' width='16' height='16'%3E%3Cpath d='M10 6v2H5v11h11v-5h2v6a1 1 0 0 1-1 1H4a1 1 0 0 1-1-1V7a1 1 0 0 1 1-1h6zm11-3v8h-2V6.413l-7.793 7.794-1.414-1.414L17.585 5H13V3h8z' fill='rgba(50,152,219,1)'/%3E%3C/svg%3E");
	mask-position: center;
	mask-repeat: no-repeat;
	background-color: currentColor;
    margin-left: 0.2rem;
}

.entry-content .epc-external-link {
	position: relative;
}

.entry-content .epc-external-link::before {
	text-transform: none;
	letter-spacing: 0;
	font-weight: normal;
	font-family: var(--wp--preset--font-family--system);
	font-size: var(--wp--preset--font-size--small);
	line-height: var(--wp--custom--typography--line-height--small);
    content:  '↪ ' attr(href);
    position: absolute;
    top: -2rem;
    background-color: transparent;
    color: transparent;
    border: thin solid transparent;
    border-radius: .3rem;
    padding: .3rem .7rem .3rem .5rem;
    font-size: .9rem;
    pointer-events: none;
    user-select: none;
    max-width: 15rem;
 	white-space: nowrap;
  	overflow: hidden;
  	text-overflow: ellipsis;
  	transition: background-color 200ms, color 200ms, border-color 200ms;
}

.entry-content .epc-external-link:hover::before {
    background-color: var(--wp--preset--color--background);
    color: var(--wp--preset--color--foreground);
    border-color: var(--wp--preset--color--foreground);
}