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)
Other Blue Daleks Edit
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)