Very strange.
Ok, next attempt.
Yes, the fact that you only see nothing might hint at your GPU, but, frankly, as you said yourself, it is well be able to show POST data, so it should technicaly be able to show BIOS.
It is possible, though, that the BIOS is graphical (UEFI), and shows a resolution that your GPU can't display together with your monitor. Unlikely, but technically possible.
Do you have a way to connect your mobo directly to your monitor (i.e. do you have a HDMI-cable somewhere?).
If yes, there is the option to connect the monitor the mobo, and see, if something shows up. A variant of this method includes removing the GPU for that cause, if nothing shows up again after connecting themonitor directly to the mobo, but i would not start with that.
If you get into bios (that way) you migth want to experiment with the settings that dictate which GPU (onboard-add-on) is selected first.