According to HTTPS description:
Hypertext Transfer Protocol Secure (HTTPS) is a combination of Hypertext Transfer Protocol (HTTP) with SSL/TLS protocol. It provides encrypted communication and secure identification of a network web server.
And to SSL/TLS:
The TLS protocol allows client-server applications to communicate across a network in a way designed to prevent eavesdropping and tampering.
Since most protocols can be used either with or without TLS (or SSL) it is necessary to indicate to the server whether the client is making a TLS connection or not. There are two main ways of achieving this, one option is to use a different port number for TLS connections (for example port 443 for HTTPS). The other is to use the regular port number and have the client request that the server switch the connection to TLS using a protocol specific mechanism (for example STARTTLS for mail and news protocols).
From this explanations we can understand that HTTPS traffic uses 443 TCP port with encription, I mean, it is not possible to a proxy to interpret the traffic and block unwanted sites because it is encrypted.
At my company, people usually use https:// to access facebook, hotmail and another websites that are blocked by corporative proxy. So, I was wondering, it is possible to block even https trafic for specific sites using a proxy or another techique beyond and integrated with the actual proxy solution? It is possible to filter or block specific sites over https layer?