Blue Dalek

Blue Daleks were Daleks bred to be non-commissioned officers. Slightly smarter than the grey Warrior Daleks but without the higher-ranking Red Daleks, Black Daleks and Gold Daleks' levels of autonomy, they served as the equivalent of sergeants and corporals. The Eighth Doctor told Sam this when she asked about the differently coloured Daleks milling about their city, some of which were blue.

During a trial of Davros conducted to root out traitors who sided with him, Blue and Red Daleks filled the room back to the walls behind the higher ranking Daleks. In the ensuing war a Blue Dalek in Davros' resistance was in command of a detail that was setting up bombs in the lift shafts to destabilise the structure of the city's control centre building. Under fire from the Dalek Prime's forces he continued to work on priming the first bomb but as the Daleks beside him were killed he made the decision to abort the countdown and detonate the bombs. The structure was weakened but remained intact. (PROSE: War of the Daleks)

Some Humanised Daleks with blue casings were discovered on the planet Kyrol. (COMIC: Children of the Revolution)

Other Blue Daleks
The Dalek Time Controller had a blue casing. (AUDIO: To the Death, X and the Daleks)

During the Last Great Time War, the Dalek members of the Eternity Circle were mostly blue, with silver domes and globes. Other Daleks present at the Daleks' temporary base in the city of Andor on Moldox were silver with blue domes. (PROSE: Engines of War)

After the Time War, Strategist Daleks of the New Dalek Paradigm were identified by their blue casing. (TV: Victory of the Daleks)

Behind the scenes

 * Dapol, which produced a range of toys based on the classic series of Doctor Who, released several original blue Dalek variants which did not appear on-screen.
 * W008-MOSV was blue with gold details including a claw manipulator arm.
 * A "Millennium Dalek" (W008-M1) had a sparkly blue body with a golden neck grille, slats and sense globes.
 * The Dalek Time Controller was originally promoted as being red (DWM 411) before appearing on audio covers.
 * In episodes 3 and 5 of The Chase, some of the Dalek props recognisable by their dark domes were loaned from the first Peter Cushing movie Dr. Who and the Daleks, but the story being Black & White makes it unclear if they were intended to remain blue.
 * A sky blue Dalek with silver sense globes was made at the Madame Tussauds' Doctor Who exhibition and appeared on the back of the Radio Times 20th Anniversary Special. Another partly sky blue Dalek appeared on Pebble Mill at One with Peter Davison cast as the new Doctor.
 * For the Video Gift Set release of The Five Doctors, an exclusive Dapol action figure was made similar to the Tussauds Dalek color scheme.
 * In The Doctor and the Dalek, the player has the option to turn Lumpy's casing blue, among other colours, during simulations. However, it reverts to its original bronze colour after the simulation is completed.
 * In Dalek Hack, the player has the option to turn a Dalek's casing blue, shaped identically to the bronze Dalek but matching the Strategist Dalek of the New Dalek Paradigm.