This can be caused by friction and as the walls of veins and the subcutaneous fat becomes thinner with age, they can bleed easily with minor trauma and cause such blood blisters.
If this does not itch / burn / hurt in any way, you do not need to be worried about this.
You can protect this area from frction to prevent formation of more such blisters.
Do not break these, they should go away on their own.
Kind Regards, Dr.Joshi
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