I have a blue lump that has appeared on the back of my knee . it may b a vein but not sure . it's the size of a grape . it's sore .

Baker's cyst: You describe a small baker's cyst which is a small heniation of the joint capsule through to the back of the knee. Usually, we don't do anything for it unless it becomes significantly painful.

Sounds like a vein.: Veins are usually blue in color and commonly can form behind the knee. Varicose veins typically appear grape-like. If it is hard and tender. It could also be a clotted vein. You should see a vein specialist for an evaluation.