General block name search
One can e.g. use the general search in the /wp-admin backend to find some text within the post's content that stores the blocks:
We can search for a prefixed block name wp:my-plugin/my-block within the posts:
where we have an extra space (url encoded as ...
The easiest way is probably to make sure the admin bar is active on the front end and use the "edit post/page" menu item to get to the editor of that page.
.... but then it might be that the specific page is generate by code and in this case you will not be able to find it anywhere in the admin and will have to hunt for it in the code.