Are you sure the ?discuss=1 query param isn’t the cause of the issue?
Are there any security permissions on your blog category, or is it set to allow the group ‘everyone’ to Create / Reply / See?
What version of Discourse is your site on?
Also, what is the full error message that you see after the Failed to execute postMessage on DOMWindow text? I would expect it to be something like The target origin provided (<target_url>) does not match the recipient window's origin (<origin_url>).