Photoshop 21.2.3 + macOS 10.15.7 on a 2020 16" MacBook Pro here and my screen goes black for a couple seconds when launching Photoshop whether the launching Photoshop is the foreground app or not. Deleting the UXP folder had no effect. I was already running with "Auto-show Home Screen" disabled and "Use Legacy New Document" enabled. What appears to have worked for me was disabling "Use Graphics Processor". Thanks for the tip Akash!
Turns out disabling "Use Graphics Processor" introduces a slew of other bugs while actually using Photoshop (tabbing away from the app turns the canvas the default gray color until you zoom or otherwise interact with the canvas, when making a selection with the marquee tool random sides of the marching ants selection rectangle disappear and reappear, something inside my laptop makes grinding sounds while drag-resizing the window, etc) so it's back to the heartstopping black screen on launch for me.