The Blue Chrono-Dalek was capable of acting alone and following the Daleks' enemies through time; (TV: Susan and the Daleks) time travel at that point in Dalek history was in its infancy, with its existence being beyond the understanding of ordinary drones. (AUDIO: The Mutant Phase) After an older version of Susan Foreman (TV: Susan and the Daleks) than the one who would eventually help foil the invasion altogether and settle there for a time with David Campbell (TV: The Dalek Invasion of Earth) was noticed intruding on Dalek-occupied London by the Red Dalek and a Silver Dalek drone on patrol, she ran back to the Doctor's TARDIS and flew it to a different time-zone.
However, the Red Dalek summoned a Blue Chrono-Dalek. A few minutes after Susan had landed in a different time-zone, where she'd accidentally let the TARDIS be seen by a child and had invited him into the console room, the Blue Chrono-Dalek appeared and entered through the TARDIS's open doors, declaring that the time machine now belonged to the Daleks. (TV: Susan and the Daleks) Susan survived this encounter. (AUDIO: An Earthly Child, etc.)
Behind the scenes
Although the minisode Susan and the Daleks (included as part of the VHS special release More than 30 Years in the TARDIS) was the only appearance of a "Blue Chrono-Dalek" by that name, Big Finish Doctor Who audio stories later introduced a figure known as the Dalek Time Controller, who, in addition to being the Daleks' expert on time travel, also had a blue casing. There are no known supported evidence that they are the same Dalek.
According to the end credits of More than 30 Years in the TARDIS, Michael Wisher voiced this Dalek (as well as all the other Daleks in other minisodes throughout the documentary), having previously voiced the Daleks during the Jon Pertwee era, as well as for a single scene from Genesis of the Daleks, although uncredited.