The Blanks were humans whose minds had been wiped of all memories and personality by the Talerians. (PROSE: Apollo 23)
Characteristics
Professor Charles Jackson's original intention for the Blanks was to replace the minds of dangerous prisoners with the dying so that the dying could live on. To accomplish this, Jackson would erase all of their memories and personalities, effectively "blanking" them for a new mind to be downloaded into. He would also keep a back-up copy of the original minds in the data storage of the base. During the Talerian invasion, the Blanks would instead be used to house the minds of Talerians brought in by Androparg.
Blanks and anyone housing a Talerian mind had pale gray eyes.
Those who weren't given new minds immediately were instead turned into effective puppets for the Talerian's use. In these cases, the original personality of the person would be downloaded back into the person's body so they would continue on as normal until they were needed. Once the Blanks were given a programed instruction, their personality would disappear, and they would become mindless puppets that only existed to follow the instructions programed within them. Due to their mindless state, the Blanks would focus on only their task to the exclusion of all else. After completing their tasks, the Blanks original personality would return with no memory of what they did while under the control of their programing.
Blanks with their original personality still present but deactivated, seemed to have a slight degree of awareness and extreme situations could cause the mind to reassert itself, at least briefly, over the programing. When Marty Garrett accidentally caused an explosive decompression that killed him, the real Garrett appeared to resurface for a moment, causing his eyes to turn his original pale blue and for Garrett to show surprise, pain, and fear before being blown out into space. Despite seemingly having been blanked completely with only programing left, Amy Pond suddenly resumed her previous command of walking, distracting Lars Gregman for the Doctor and Major Andrea Carlisle. The Doctor suggested that if the Blanks' original personality wasn't totally erased, then perhaps something remained deep down waiting for something to latch onto to reassert itself. When the Talerian possessing Gregman was knocked out, it briefly dislodged the Talerian's control of the body, causing the Talerian to think that he was really Gregman for a moment before the Talerian consciousness flowed back into the emptiness of Gregman's mind and he remembered everything.
Exposure to the back-up copies of the original mind completely erased both the Blank programing and any downloaded Talerian minds and allowed the original personality to be fully restored. ({{PROSE]]: Apollo 23 [+]Justin Richards, BBC New Series Adventures (BBC Books, 2010).)
History
The Blanks were first created on Base Diana by Professor Charles Jackson as part of his plan to replace the minds of dangerous prisoners with those of the dying so the dying could live on. In order to do this, Jackson would erase all of their memories and personalities, effectively "blanking" them for a new mind to be downloaded into.
After the Talerian Androparg possessed Jackson, he began turning the personnel of Base Diana into Blanks with the purpose of downloading Talerian minds in instead of humans for a Talerian invasion of Earth. Some of the people he blanked did not receive Talerian minds immediately and were instead turned into effective puppets.
After the arrival of the Eleventh Doctor and Amy Pond, Amy witnessed the actions of several Blanks and was turned into one herself. By this point, only a few people in Base Diana weren't Blanks or Talerians with most completely unaware of it as their original personality was downloaded back in and active when they weren't needed as puppets or home for new Talerian minds. As Amy's original personality wasn't needed, it not restored as it was not needed for her mission to learn of the Doctor's plans to stop the Talerians and thus when she was not acting under her programing, she was mindless. Using optical stimulation from the sonic screwdriver, the Doctor was able to give Amy simple commands even while she was mindless as they retrieved her back-up, although Amy displayed a moment where her true personality seemed to shine through for a moment, causing her to resume her previous command of walking, making her act as a distraction against Lars Gregman for the Doctor and Major Andrea Carlisle. Following the Doctor's orders to keep her back-up close and safe, Amy drank it, allowing her brain to extract the imprint of her mind from the water, erasing her Blank programing and restoring the original Amy.
With the knowledge that contact with water from their back-ups would restore the Blanks to normal, the Doctor had Carlisle lock all of the doors of Base Diana open and rig the fire suppression system so that it could not be turned off outside of the control room she controlled. The Doctor then filled the fire suppression system's tank with water from Base Diana's reservoir and the back-ups of the Base Diana personnel before triggering the system. Contact with the water immediately erased the Blanks programing and the Talerian minds in those who had already had one uploaded. The Base Diana personnel also blanked out again for awhile, but once they regained consciousness, they had their original minds back with their Blank programing gone and no knowledge of their actions as Blanks. (PROSE: Apollo 23)