The screen is always dark during the countdown (live view), but the finished photo is displayed correctly.

This is caused by an aperture that is closed too far or ISO values that are too low. In live view the camera shows the ambient light without flash, so the image looks dark, while the final shot with flash is correctly exposed.

Set the aperture to f/5.6 to f/8 and raise the ISO (up to about 8000, depending on the camera). Then reduce the flash power so the final photo is not overexposed. The matching basic values are also given in the FAQ Lighting behind the guests.