If you do pacifist then you restart, do "take out your weapon" and when you finish the fight, you get a ending. Hooray.
For me, the ending isn't any different from the normal genocide ending (when you kill Lu first) . . . :|
Oof i did that days ago i know how to do it, go pacifist and when blue is sparing you, KILL HIM. then you get an ending
Fight him again after and it says "Blue Reappeared! (He's Pissed)"
yep!