Blue Girl is a robot that Al made at one point.
It is unknown when or why Al made Blue Girl, but she first appears in Fallout 4 Quest Mods: Tina's Cookiepocolypse - 3 where she participates in the defense of Sanctuary Hills against Feral Ghouls that came when Al summoned an Eyebot named Sir Reginald to warp in a Power Pylon. When Tiny Tina teleported her Assaultron Zealots to help, they accidentally attack some of the settlers, Blue Girl included. Al noticed this and attacked a Zealot, which lead to his many deaths.
She appears as a cameo when Al was introducing a mod that adds Mister Cuddlesworth in Fallout 4. She was walking behind Al as he was talking to Mister Cuddlesworth.
She makes a clear full-body appearance in Fallout 4 Mods Week 68 - Legion Assassins and Rideable Deathclaw! when Al reviews the Quiet Robots mod and approaches robots to demonstrate the mod.