Can unexplained blue skin be caused by ventricular septal defects?

Possibly: In the setting of pulmonary hypertension and the shunt reversing to right to left (vsd flow is normally left to right) you can have deoxygenated blood going to the aorta and eventually the vessels at the skin. If you have such significant pulmonary hypertension that this occurs you certainly have many additional symptoms!

Rarely: A large vsd that is untreated can lead to the development of pulmonary hypertension during childhood. It worsens over time and ultimately results in shunting of blood from the right ventricle through the vsd into the aorta and causing cyanosis (blue skin).