once a swap!sans called blueberry, his au was almost completely wiped out by error except both sans and papyrus. for unknown reasons error has decided to abduct Blue rather than
kill him, leaving papyrus to be found by CORE!Frisk and be taken to the omega timeline. Over
time Error becomes more accustomed to Blue's company, eventually showing him a Transdimensional window of his first
time leaving the anti-void as well as his first
kill, this, in turn, makes blue extremely uncomfortable, leading him to say "I'm still your
friend, error. despite this. ok?". deeming this as manipulation, error snaps at blue and confronts him on what he calls "fake
friendship", and shows him the soul of his
human and explains the concept of resets, explaining how he didn't just randomly attack his au, but rather observed his au waiting for the right chance to strike, error then attempts to
get blue to fight him, resulting in blue believing that error doesn't want him to right them and that he has found errors weakness, that being that he is scared of friendship. this assumption angers error so much that he abandons blue in the anti-void with the human soul and as a result of being trapped in the anti-void for so
long. blueberry became an error.