Hello~I would ask a dermatologist if you are concerned about this, it is probably nothing, but, they can tell you for sure and set your mind at ease. Sometimes this type of discoloration can be due to certain allergies and even heat, sort of like a heat rash.
Have you been to a dermatologist or podiatrist? Sometimes certain types of skin cancer and benign dermatological problems can appear on the soles of the feet. You should see a doctor soon to make sure that this is not something serious.
I have the same spots on the bottom of my feet; I am French-American with a mix of this and that too, not Afro-American though. My physician assessed them and told me they were hyperpigmentation and not to worry about it. I guess it is no big deal unless you are working as a foot model. They appeared after I turned 39.
You can consult a dermatologist and see what he/she recommends to use to fade them, etc. They don't bother me being there and no one else can see them except my husband so, it is not a big deal for me.