More information from Anime News Network:
In the story, the title character received major injuries in an accidental explosion when he was 9, and his mother was put in an artificially induced coma. The new special will depict how the Black Jack character became an unlicensed doctor and how his destined rivalry with Dr. Kiriko came to be.Looks interesting and it's always great to see new Black Jack stuff, though for the sake of the character I wish he had even just a touch of silver in his hair.
Naho Toda (Romance of Three Kingdoms, Oblivion Island: Haruka and the Magic Mirror) will play Black Jack's mother, and Risa Naka (anime and live-action The Girl Who Leapt Through Time) will play a fellow medical student. Yukiyoshi Azawa and Masachika Ichimura will also co-star.