Dark knuckles and dark knees and elbows are one of the most stubborn areas to deal with. They take the longest time to respond to treatments which is a big headache when you're trying to even out your body's tone.
There are a couple of reasons that can cause dark knuckles including acanthosis nigricans, a skin condition that commonly occurs in darker-skinned people, health conditions like Polycystic Ovarian Syndrome, diabetes and vitamin deficiencies.