Our Campaign API allows the creation of a custom HTML email regardless of what the source of the content is. This means that you (or someone) could create a script that takes the content of your blog post and submits it along with the other required data such as the email subject and list selection to our API. This will add new custom HTML email in your account with the blog content as the body of your email.
Once the email is created you would not be able to use the API to send it but instead would need to go into your account and schedule it to be sent.
I am not sure how you would integrate this directly into Wordpress as I have never written a plugin for it however the functionality of dynamically creating the email is possible with our API and should not be terribly complicated to develop.
Please let me know if you have any other questions.