That sounds like a video driver issue and O/S not working correctly.

The screen goes black for a second and then... everything is normal after that, or it stays black until you escape out of full screen?

It's normal for the screen to black for a little under half a second. There is a "transition" built in for going into and out of full screen. Not sure why. You could experiment with the setting if you feel it's too long:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste transit and pause while the list is filtered

(3) Double-click the full-screen-api.transition-duration.enter preference to display a dialog where you can enter the desired values. Mine shows:

200 200

That represents 200ms fade to black and 200ms fade back from black.

There are 1000ms in a second, so that is two tenths of a second for each fade, or a total of four tenths of a second.

To make the transition more of a blink, you could try

50 50

or to get rid of it completely, try

0 0

Does anything look like the best amount of time to your eyes?

(4) Double-click the full-screen-api.transition-duration.leave preference to set the same values, or it could be weird coming out of full screen through a different transition.

Note: If any preference becomes messed up, you can right-click > Reset it to restore the default value.