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The Blacklight was a key item used in the Hello Neighbor Pre-Alpha. It was used to get the number code for the Keypad on the basement door, by shining it on the wall behind one of the paintings in the hallway by the backdoor would show the code.
To obtain the Blacklight, you first have to use the brown, rusty heart key on the padlock and use the hammer to take the board off the doors, and then go asleep. After you wake up, the Blacklight will appear on the dresser next to the neighbor's bed in his bedroom. Other times the Blacklight will appear on the neighbor's bathroom sink. After you obtain it, the player can go out into the hallway, go by the backdoor, and pull the picture of the neighbor and his friend off the wall. Then, you turn the Blacklight on and shine it on the wall where the painting once was; there will be a purple glow on the wall. And you've found the code. Input the code on the keypad by the basement door, and you'll have access into the basement.
The Blacklight nor Keypad did not appear in future versions, as the Keypad was replaced with a Keycard reader. Interestingly though, the Keypad, it's number components, and the Blacklight reappeared in the Alpha 3 files. (Video link: https://www.youtube.com/watch?v=rAYYpA1cTIs at the 7:00 mark and the 14:46 mark.) It's unclear why it showed up in the files, but it might have been planned to come back for the basement door. The Blacklight and Keypad never came back in the files after Alpha 3, killing it off for good.