Recommended apache config for Ghost(Pro) subdirectory proxy. SSL is required.

apache.cnf

# You'll need to enable the ssl, proxy and proxy_http modules:
# sudo a2enmod ssl proxy proxy_http

<VirtualHost *:80>
        ServerName <yourdomain.com>; # Replace with your domain
        Redirect permanent / https://<yourdomain.com>  # Replace with your domain
</VirtualHost>
<VirtualHost *:443>
	ServerName <yourdomain.com>; # Replace with your domain
        
        SSLProxyEngine on

        # Your SSL Certificate details. Replace with your own SSL setup
	# SSLCertificateFile /path/to/cert
	# SSLCertificateKeyFile /path/to/key;

        <Location "/blog">
                # Replace with your subdomain, https matters here
        	ProxyPass https://<yoursubdomain>.ghost.io/blog/
	        ProxyPassReverse https://<yoursubdomain>.ghost.io/blog/ 
        </Location>
</VirtualHost>