I have kubuntu 13.10 installed on my desktop, cpu Q8400 with Radeon HD5800 card. I have got the fglrx installed from the package manager which is ATI driver packaging version 13.10 since I wasn't able to get the latest 13.12 version downloaded from ATI website to work on kubuntu 13.10 (failed to compiled properly due to some software packages missing).My graphic card has both DVI (which I use a DVI to VGA converter) and a HDMI port.
My problem is same as yours, it give me a blank screen if I use the HDMI port to connect to my monitor.
However, I found that if I connect to another monitor with the DVI port at the same time, all with be fine. Both the monitors will show cloned dual images.
This blank screen problem will persist even if I go to the ATI catalyst configuration and disabled the DVI monitor; as long as the DVI port is plugged in (even if it is disabled in the ATI config) the HDMI monitor will show desktop screen without problem. But when I removed the DVI port connection, the HDMI monitor will show a blank screen.
Btw, if I purge the fglrx driver and used the default free ubuntu graphic driver this problem goes away. So, the graphic card has no problem. The problem lies with the ATI Radeon proprietary driver. However, I need the fglrx driver for the HDMI audio and the flash player to work properly.
Thus, now I am using my desktop by connecting to both monitors but with the second monitor(DVI) power off and disabled at the ATI config . Strange but it works for me. hahaha
Hope this help you.