There is another related topic here: Add option to set canonical_url to embed_url. If what you are wanting is to have the canonical URL of the Discourse topic set to the URL of the embedded blog post, it would be best to continue that discussion.
I think the biggest issue with implementing this is what to do about this issue that’s raised in the OP:
Should the blog post’s URL only be used as canonical for N pages presented to the search bot? After all, only a certain amount of posts is embedded in the blog post. (N probably should be 1)