What is the stock block color for a 1986 Chevy silverado 350 big block?

There's no single "stock block color" for a 1986 Chevy Silverado 350 big block. The color varied depending on the foundry and the year of production within that year. While many were a darkish gray or a sort of battleship gray, some could be lighter or even have a slightly greenish tint. There's no official GM color code for the bare cast iron block itself.