That's the Lock Screen. You can't really get rid of it unless you leave the phone completely unlocked (not a good idea). You can however prevent the notifications from being shown on the Lock Screen. Go to Settings -> Security & location -> Lock screen preferences and select 'On the lock screen' - from there you can decide what you want shown on the Lock Screen.
This is how I have it set now.
I used to hit finger print unlock and I would be in my phone. Now, sometimes after the phone has sat inactive for a while, I hit finger print unlock and get that night mountain view, I then need to swipe it up to get to my phone. So two steps to get in the phone, very annoying.
It is odd that the 'Recents' button isn't shown but I've seen other apps suppress it.
What happens if you press the 'Home' button when that black screen is shown? If you are returned to the Home screen does the 'Recents' button reappear? If it does try using it to see a list of the running applications - perhaps that'll give you a clue as to which application is causing your problem.
I was sitting here just now doing nothing with the phone on my desk stand. A notification came in - a basketball game score and new email notification. The notifications did not show up, just that black screen.
I pressed the home button and got and overlay of the cover of an audible book I was listening to at the gym with notifications on it and then had to do the fingerprint unlock.
I have the dark moon screen wall paper. You can see the stars at the bottom of the first screen shot. I am going to switch to a lighter wallpaper and see if I can get a better idea what the black object is on my screen.