I just fixed mine. Turned out it was a bad solder joint that I had missed when I was checking the PCB.
Have you checked the capacitors on yours? the hints I've gotten from others is to look at the _KBRESET signal to see if there might be a dried up/bad capacitor holding that signal down.
I have the exact same problem. Hi Tek keyboard connected = black screen. Any other working keyboard = fully functional Amiga 500. Have you gotten any further in your troubleshooting?