Papyrus fight glitch or something
Solution 1:
Have you attempted either the FIGHT or MERCY option during your fight?
In my experience, many return players remember choosing to flirt with Papyrus as the trigger for the second phase of the fight. However, the Flirt dialogue isn't enough to progress the fight to the second stage - he won't do a blue attack until you've attempted either FIGHT or MERCY then SPARE, so if you continually select ACT or ITEM (like the Flirt or Insult dialogues) menu options the fight will stay on the red soul phase and never progress.