The bride-to-be that appeared in the TARDIS is Donna. She is not the Doctors new companion, just here for this one episode.
The Doctor's new companion is a black girl. The character's name is Martha Jones and she is played by Freema Agyeman. She (Freema) had a minor part in the episode "Army of Ghosts", but not as Martha Jones.
By the way, the Christmas special was great. I am anxious for the new season to begin.