Block rendering filter

add_filter( 'render_block', 'dev__paragraph_add_block', 10, 2 );

function dev__paragraph_add_block( $block_content, $block ){

	if ( 'core/paragraph' === $block['blockName'] ){
		$block_content = new WP_HTML_Tag_Processor( $block_content );
        $block_content->next_tag(); /* first tag should always be ul or ol */
        $block_content->add_class( 'wp-block-paragraph' );
        $block_content->get_updated_html();
	}

	return $block_content;

}