Where on your foot are they? Any additional details will help us answer you better.
If they are on the bottom of your feet it could be plantar's warts.
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Otherwise, they could be from dye in new socks, or from a rivet in a shoe that has discolored the skin.
I am not sure what they could be at this point. I would just observe them. If they get larger or don't go away, it could be small blood blisters possibly from shoes that fit wrong. If you are on your feet a lot, that could be a possibility.
Usually plantar's warts are on the bottom of the foot.
I will continue researching, and let you know if I come up with anything else. In the mean time perhaps another expert can help.
Blood blisters are the same as blisters. Only instead of filling with clear fluid they fill with blood.
Thanks for the accept!