Thanks for the help. I'll start moisturizing and soaking the foot today.
Hi
It seems that the dark patch has developed due to friction from your foot-wear. You should try applying a moisturizer, Vaseline petroleum jelly and soaking your foot in warm water for a few days which will soften the skin and the discoloration would go away.
If it doesn’t resolve in a few days time, you should consult a doctor. Hyperpigmentation may also occur due to aging, sunburn, melasma, pregnancy, skin trauma, side effect of medication, use of birth control pills, or hormone replacement therapy. Various systemic diseases such as Addison's disease, hemochromatosis, primary biliary cirrhosis are associated with hyperpigmentation.
Take care