Do you rest your on arm on the elbow while working on the laptop? I feel what you have is very physiological. Everyone has thicker and a darker elbow skin naturally. Its the body's way of protecting. But when it gets darker abnormally then it could be a part of a condition called Pseudoacanthosis Nigricans . In which case, modify your lifestyle. Eat a healthy and balanced diet , sleep on time by 11pm latest, wake up early and exercise. It helps in reducing such joint pigmentation. And use plain coconut oil on he elbow before you take a bath. It softens the skin.