Blacked out images with flash

[maxxum7d]

I bought an extension cord and shoe for my kit and tried it a couple of times. The flash fires, but the resulting image is dark, as if there was no flash. The funny thing is, I pointed the flash directly towards the camera so I can see it through the viewfinder, and I can see it go off before the shutter clicks. And the preview from this experiment? No flash! It seems that the flash fires off a fraction of a second before actual exposure by my camera.

If I put my flash back onto the camera, it shows correct exposure (HSS works as well). Any hints?