Try this PHP Snippet:
add_filter( 'render_block', function( $block_content, $block ) {
if (
!is_admin()
&& 'link' === get_post_format()
&& ! empty( $block['attrs']['className'] )
&& 'my-custom-class' === $block['attrs']['className']
) {
$block_content = str_replace( '<a ', '<a target="_blank" ', $block_content );
}
return $block_content;
}, 10, 2 );
NOTE: this line: && 'my-custom-class' === $block['attrs']['className'] ) { change the my-custom-class to your own class. Then select the Headline block that has the link applied, and add that class to Advanced > Additional CSS Class(es)