Hey all! I was looking for a solution for this as well, and I think I've found it! If you click on the Share tab of the Blog Post Settings there is a Create Email Draft option (see my screenshot) and when you click that it imports the post content into a draft email campaign. You can edit the email, and then send.
It would be really nice if it could be automated though, @Squarespace!
But it's better than nothing?