The problem is that the phone might have a shortage or breakage in the LCD screen. The LCD holds a thin line in your phone. If you have broken it, A black dot may appear and grow within days. My advice is to leave it alone for a while. Let it rest for some time. You can get it fixed by replacing the screen of some sort. It costs I don’t know how much for a IPhone or a Android. I have experienced it as my friend recommended me to leave it alone. And to NOT let it spread. If it goes to the whole screen, Most likely, It’s game over. So, That’s my advice.
The reason you cannot fix it is because your LCD screen is probably broken from underneath. The black spots are ink. So, I have this on my IPhone. I let it rest, I DO NOT RECOMMEND TRYING TO FIX IT YOURSELF. It spreads more than you can think. Try letting it rest and see if it spreads, The bad news is that, It probably will stay there forever. But a tip is that, To not drop your phone or it will make it worse.