From what I understand, the black/devil riders were a group of German soldiers or mercenaries who were famous for using black painted armor. They developed various techniques for fighting on horseback, especially using pistol-like weapons. What is the origin of the black riders, and were they a highly renowned group of mercenaries like the Swiss pikemen mercenaries were?
You are referring to the Schwarze Reiters (i.e. black riders), named after the dark armour they wore. This was a type of cavalry that appeared in Germany after the decline of the medieval lancers, but later became a generic name (usually shortened to just reiters) for German cavalry mercenaries.
Around the mid-16th century, advancements in firearms as well as proliferation of pikes were making the earlier heavy cavalry obsolete. One of the ways cavalry forces responded was to shift their focus to mobility and firepower. The Schwarze reiters were the pioneering example of this paradigm shift. Created by Günther of Schwarzburg, the black riders adopted pistols as their primary weapons. Their integration of firearms into cavalry manoeuvres developed into the famous tactics known as the caracole.
The Schwarzern Reitern was probably so dressed and thus named due to the association of black with elite military status, and/or because of Schwarzburg (i.e. black castle).