My son has on and off dark bluish-black patches on his left hand palm since three years. Why?

Hi doctor,

My son is six years old. I have two questions for two patches on his body. For three years, we have seen this patch on his left-hand palm, which appeared dark bluish-black than it is today as it appears in the photo attached, but of the same size, as seen today and then gradually in ten days, it subsides. And then the skin would appear normal in color on that patch. We first thought he has got hurt while playing, but this has happened 3 to 4 times so far in three years, and it is on the same spot. For two months, it is seen as a light-colored patch as in the attached photo, size is the same as before, but this time it did not subside. On touch, it feels like the skin under it is thickened. But it does not cause him any pain, itch at all, even if pressed on that patch. I want to understand what it could be, any tests we need to do? Also, there is a patch we have suddenly noticed on his left hand, underarm five days back. First, we thought it is a sketch pen mark as he does painting. But it did not vanish. We noticed it just five days ago. 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?