Nothing to do with dropping (the one I have suffer this but was never dropped but used) nor handling nor age (the other one was very old but not used and was in perfect condition).
It is just the weakness all(?) N950 displays suffer.
It may be the result from usage or just material flaw.

Maybe someone knows it better?

My black spots are more like dotted liney in the middle. Others had them like this one here. So maybe hard pressure (from resistive N900 users? ).

someone had told me that it can be fixed but didn't tell me how...

Well, I am not, but I think my nickname says where I am from

...and at some point I lost you bro...

I found this vid, it might work?

https://youtu.be/X_sn6yU5tao

Wow! interesting, but quite risky... Some comments say that it made it worse... And in my case, it is a prototype... hmm...

Didn’t look at the comments, though if some experienced it getting worse, it might not be the way to go with a prototype

I had the black creep on an N9 and removed it completely with repetitive pushing on the screen similar to the video (but with fingers). Not sure the one on the prototype is the same as it has very clear shape and edges.

Rubbing the display for 2 hours to push the liquid or whatever to the corner where it had "escaped" ?? whoo...!

I somehow feel that is not a very permanent fix; if it works at all it might well get back like it was or worse after some time.
(and might be dependent on the display type and blotch location, can it be done at all if the blot is not in the corner?)

As for doing that on N9, no way it can help, N9's got an OLED display which does not contain the liquid layer that the LCD of N950 has.
(And hence there can be no blotching on N9 screens AFAIK)

Might have been N950 too - it was too long ago.

Well, still I don't want to risk it... Some comments below the video mention that it got even worse... Maybe it works in some cases, eg if the spot is new or depending on the screen etc...