I think if you have a bad itch, it is understandable that you scratch it. Don't be hard on yourself. You may have gotten a bit rough and obviously didn't realize you were causing an issue. There is something called Post Inflammatory Hyperpigmentation which it sounds like has happened to you. You itch until the skin turns darker. Topical depigmenting agents, such as hydroquinone, azelaic acid, kojic acid, licorice extract, and retinoids, can be effective alone or in combination with other agents, and procedures such as chemexfoliation and laser therapy can also be incorporated into the management strategy if needed. That's the usual treatment. Fade creams may help. Do you have a wide selection of products at your local drug stores? Don't be ashamed. People have all sorts of skin issues and dark marks on feet will not upset anyone. :>)
Hello~Try using a good, moisturizing cream, I think there is one actually made for dry feet in the pharmacy, your pharmacist would know for sure. Also, soak your feet in warm Epsom salt water, this is not only relaxing, but also will help with the issue you are having.