If one's tefillin have tiny white spots on the corners or edges of the batim, do these need to be painted black? Or must the spots be a certain size before anything must be done about them?
Tiny spots, or even larger ones, on the batim do not invalidate the tefillin. (MB 32:184
However, white spots on the straps, especially on the first loop which fastens the bayit to your arm do and must be fixed. These are very common and most people don't notice them right away. (Mishna Berurah 33:3:19)
I had that question for a Sofer once. The psak was that if the dot is less than a millimeter "or so" in isolation, then it is fine. In this specific case he looked at the Tefillin to determine, but that was the principle he was stating.
It may be mehudar in terms of making it look nice for the Mitzvah to make it completely black even if the dot itself is of no significance.