My son has a dark spot on his left palm and it recurs often. What could it be?

Hello doctor,

My son is six years old. For three years, we have seen this spot on his left hand's palm that appear very dark black-bluish than it is today as it appears in the photo. And then it would suppress, and the skin would become normal color. It used to be of the same size as it appears now. Just the color would be dark initially and then subside. This has happed three to four times so far in three years. But now since two months, it is seen as light color (greyish/blackish-blue) patch, size is the same as before, but now it did not subside. On touch, it feels like the skin under it is thickened. But it does not cause him any pain, itching at all even if pressed on that patch. I want to understand what it could be, any tests we need to do? Also, a patch we have suddenly noticed on his left hand, underarm. First, we thought it is a sketch pen mark as he does painting. But it did not vanish. We noticed it just three to four days back. He does not feel any pain, discomfort, or itch on that patch. This patch does not have any skin thickness and has never been before on that spot underarm. Is there any further investigation required for these two patches? What could it be?