HDMI has been working fine for as long as I remember. However, today when trying to connect it to my Sony TV, the HDMI wasn't working. It's not the cable or TV (tested different laptops/cables) so it must be my MacBook Pro with Retina Display, Mid 2012.
I've had some issues with black screens when booting recently (still do, but can work around it by shutting my laptop lid for one minute and then opening it and it seems to work) which may be an influence.
I tried resetting the PRAM and SMC etc., but to no avail. When I unplug the HDMI cable, my screen goes to black for a brief second so it must still detect something.