These body joints are used and get touched all day long. Rubbed dry with towels, in constant contact with clothing, they also receive bumps during activities such as sports.
The result of all this mechanical stress, chafing and movement, is friction melanosis: in some people, often having a darker skin type, elbows and knees turn a darker color than the surrounding skin1,2,3. Why there is this hyperpigmentation is still unknown. One theory is that it could be due to pigment incontinence, where epidermal melanin leaks into the dermis4.
1 Magaña, M. & Herrera-Goepfert, R. Friction hypermelanosis: other variants. J. Am. Acad. Dermatol. 47, 454; author reply 454–5 (2002).
2 Hidano, A., Mizuguchi, M. & Higaki, Y. [Friction melanosis]. Ann. Dermatol. Venereol. 111, 1063–71 (1984).
3 Wong, C. & Lin, C. Friction Amyloidosis. Int. J. Dermatol. 27, 302–307 (1988).
4 Al-Aboosi, M. M. et al. Friction melanosis: A clinical, histologic, and ultrastructural study in Jordanian patients. Int. J. Dermatol. 43, 261–264 (2004)