The Black Knight (ブラックナイト) was an unused class in Fire Emblem: Genealogy of the Holy War. The class is virtually identical to the Great Knight class and is only alluded to in the form of concept art. The main difference between this class and the Great Knight class is the latter can only use axes in Genealogy of the Holy War while the Black Knight concept artwork is seen using a Lance.
Enemy Great Knights such as Schmidt also use a Black color scheme identical to the Black Knight concept art. Thus it is reasonable to say this class evolved into the Great Knight.