Hex Socket Button Head Bolts are a type of screw that is commonly used in furniture, machinery, and construction. They have a hexagonal socket head that allows them to be tightened with a wrench or socket, and they come in a variety of sizes and materials. 
Black oxide surface finish (aka blackening), which is a coating applied for mild corrosion resistance in dry environments. Black oxide treated metals/fasteners can rust in the right environments, so it should be stored properly to avoid rusting.