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.
Rat snakes are one of the largest kinds of species of snakes, with over 50 types of sub-species. One of the more common kinds of sub-species is the yellow rate snake. The yellow rate snake can reach up to ten feet but, in general, will grow as big as five to seven feet. These snakes are nonvenomous; however, there have been cases where some yellow rates snakes have been found to contain small doses of venom.