Can you tell me if it's dangerous to take a blood sample from the superficial leg veins?

Should be safe. Blood drawing is usually done from hand and arm veins because it is technically easier and more accessible. There are occasions when blood is drawn from both superficial or deep leg veins but these carry a higher risk of potential complications especially phlebitis (superficial or deep). As a precaution, blood is only drawn from the leg veins if no hand veins are available.