The blue colour is the flow of blood from right ventricle (RV) across the ventricular septal defect into the over-riding aorta. This causes desaturation of aortic blood and cyanosis in Tetralogy of Fallot. The blood from the right ventricle preferentially enters the aorta which is over-riding the ventricular septal defect (VSD) because the right ventricular outflow tract is narrowed in Tetralogy of Fallot as result of infundibular stenosis. The live colour Doppler video below shows the blue flow from RV to aorta across the VSD. Encoding in colour Doppler is blue for flow away from the transducer (located at the top of the sector) and red for flow towards the transducer.