Blackbox first appeared in July 1997, when BradleyHughes, who was a student in Amarillo, Texas, USA, at the time, decided to write a window manager for X. The original purpose behind Blackbox was very simple: he just wanted to see if he could write a window manager and make a significant and groundbreaking contribution to technological/academic community.
He did so, using C++ and completely original code. He was admittedly inspired in the writing of the graphics class by the WindowMaker? code, which also impressed him with its clarity and simplicity, making him decide to follow these characteristics.
As Blackbox evolved into a usable window manager, the focus was shifted. Blackbox was now meant to be small, fast and simple, and have no (mandatory) dependencies other than X itself.
You can find clues to the manner in which Blackbox evolved in the changelogs kept in the BlackboxCVS:
It may be interesting to know that bbkeys used to be code integrated in Blackbox, but it was made into a standalone tool and taken over by Jason 'vanRijn' Kasper at some point.