Zooming on External Monitors connected to MacBook Pro make screen goes blank.

When you create an Extended Desktop display setup. you concatenate your multiple physical displays into one large Extended Desktop, and the mouse moves seamlessly across that composite Extended Desktop. The simulation is so complete, that if you park a window on the boundary between physical displays, that window behaves and scrolls "the right way" regardless of the settings of the possibly wildly different displays.

When you choose "Full Screen" on one of the displays in the group, Extended Desktop is suspended, and the non-Full screen displays that were part of that Extended Desktop a moment ago go dark, BY DESIGN. Instead, the Mac focuses everything on the One display you told it contains the information so impaortant you need the "Full [Physical] Screen" to display it.

Your Mac is functioning as designed. Extended Desktop will return to when you de-select "Full Screen".