You'll notice that the places that you see blue veins are in areas where the skin is thin and the blood vessel underneath is pushing up on it stretching the skin over it. Light hitting the dark color of the vein underneath the skin, in combination with the skin itself is what causes it to appear blue in these areas.
Common krait. The common krait (Bungarus caeruleus), also known as Indian krait or blue krait) is a species of venomous snake of the genus Bungarus found in the jungles of the Indian subcontinent. It is a member of the big four species, inflicting the most snakebites on humans in India.
Vipers (Viperidae) are a group of snakes known for their long fangs and venomous bite. Vipers include true vipers, bush vipers, rattlesnakes, pit vipers, adders and night adders. The fangs of vipers are long and hollow and enable the snake to inject venom into animals that it bites. Venom is produced by and stored in glands located at the back of the snake's upper jaw. When the snake's mouth is closed, the fangs recede into a thin membrane and fold against the roof of the snake's mouth.