What is causing my feet to go blue when sat down?

Blue discoloration in your feet when sitting down could be due to poor circulation, which might be caused by prolonged pressure on blood vessels or nerves, especially if your legs are crossed. It could also indicate a condition like peripheral artery disease or venous insufficiency. Additionally, factors such as cold temperatures or certain medical conditions affecting blood flow may contribute. If this persists or is accompanied by other symptoms, it's advisable to consult a healthcare professional for a proper evaluation.