The port showing up as blocked does not necessarily mean that your port forwarding is not working. It could just as easily mean that your machine does not have its firewall rules updated, or your application is not yet listening on 5921.
I would run netstat -na to ensure that your application is running and has port 5921 open, and I would check the (host-based) firewall log to see if the incoming packets are being rejected.
Typically VNC will not open an additional display port until the first is in traffic, so if you connect to 5920, then 5921 may open, allowing you to test your port forwarding.
pansophic