No this is not true. People sometimes say this because if you look at the veins in your arms they appear blue, and veins carry deoxygenated blood back to the lungs. So therefore people think “ah, blood with no oxygen is blue, so it must only be red when its comes into contact with air, since this has oxygen in it”.
Actually your veins look blue just because of the way light is diffused by your skin. If you were to remove someone’s skin (eurgh!) without damaging any blood vessels so the blood is still not in contact with air, then their veins would appear red. Deoxygenated blood is also a different colour than oxygenated blood in your arteries, but they’re just slightly different shades of red.