Add a do_action using the render_block filter

Block.php

<?php

namespace PT\MustUse\Blocks\CoreEmbed;

class Block
{
	public function run()
	{
		add_filter('render_block_core/embed', [$this, 'render'], 10, 2);
	}

	public function render($html, $block)
	{
		do_action('ptmustuse/blockbefore/core-embed', $html, $block);
		return $html;
	}
}