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Some days ago i took a few pictures in a park with clear sky and noticed some dust on the pictures, cleaned the back of the phone thinking some dust might be in front of camera, but the dust is inside the phone (inside lens), isn't it suposed to be dust proof or whatever. Never had this problem with any other smartphone... Im thinking of returning it. its not as resistant as sony made me belive...
The black stuff is the dust inside the camera lens.... 
return it as a this is a manufacturer defect
this kind of phones (the best of sony) costing up to 600€ can't have manufacturer defects... its not even 3 months old....
this issue was probably there to begin with, but perhaps you just notice
no it's a problem that i noticed one day, but the first pictures i took with the phone were fine. Only from a days earlier till today had this problem.
now that's quite odd, you should take it to the store asap