Dark patches on the elbows are usually caused by thickened or dry skin, friction, or sun exposure and are generally harmless. To improve them, moisturise regularly, gently exfoliate with mild scrubs or chemical exfoliants (like lactic or glycolic acid), and protect the skin from the sun. Blood pressure medicines like Cardace and Met XL usually do not cause darkening. Consult a dermatologist to rule out other causes.