Modify the rendered output of any block.

<?php

/**
 * Modify the rendered output of any block.
 *
 * @param string $block_content The normal block HTML that would be sent to the screen.
 * @param array  $block An array of data about the block, and the way the user configured it.
 */
function my_custom_render( $block_content, $block ) {

	// For the block in question, render whatever you want, and pull any attribute you need from $block['attrs'].
	if ( $block['blockName'] === 'core/image' ) {
		return 'Anything I want with any attribute value here: ' . $block['attrs']['name_of_attribute_here'] . '.';
	}

	// For any other block, just return normal block output.
	return $block_content;

}
add_filter( 'render_block', 'my_custom_render', 10, 2 );