It is important to always seek a medical evaluation by a licensed professional to ensure it is not the manifestation of something more serious. You also want to seek help sooner rather than later to prevent any further damage to the area.

You should seek medical attention right away. You did not state the size of the lesion, but you would want to exclude an acral lentiginous melanoma.

This is unusual. Please consult a dermatologist.

It is very difficult to qualify what one means specifically in terms of size, concentration, area of configuration of this darkened area to expand upon a discussion. However, I would start with your PCP and perhaps a dermatologist to have this lesion or pigmentation looked at in a timely fashion.