What are describing is an acral lick granuloma
, which is a localized skin infection made worst by licking. The black coloration is called hyperpigmentation and it is caused by inflammatory cells releasing melanin (dark pigment); the black color is not harmful, it is a consequence of chronic inflammation; it will not go away. This is what you can do:
1- First thing is to stop him from licking or it is never going to get better. Pick up an ecollar (cone) in your local pet store. Have him wear the ecollar until those lesions heal.
2- Wash the area twice a day with an antibacterial soap. If you can request from your vet, chlorhexiderm solution even better. Use the chlorhexiderm twice a day to clean the affected areas.
3- After washing as above, apply a topical antibiotic like neosporin or bacitracin. Do this twice a day for the next 7-10 days.
4- For the itch you can give Benadryl (diphenhydramine) 1mg per pound body weight every 12 hours.
5- If no significant improvement in 2-3 days, have him checked by your family veterinarian.
Hope the above helps, if you have additional questions I will be glad to answer them.