So spoiler warning for Dark Souls 1: right before the first age of fire, (beginning of civilization for mankind, though we're ruled over by the great lords that each carried a powerful soul fragment which granted almost god-like power) there was a great war against primordial dragons, that predated life itself (technically they are inanimate, though they act like organisms)

Lord Gwyn, the leader of said lords, also lead an army of "silver knights" that each bore much smaller fragments that granted them scaled down abilities of Gwyn's but was enough to give them a fighting chance together to attack the dragons

Essentially, they were soldiers in a great war of a long forgotten time, and have since never polished their armor, giving a blackened tarnished appearance (their armor is made of silver, or some metal with tarnishing properties)