It's canon.Nickame for Underswap Sans is Blue or Blueberry,only the Sanses call him that.The Underswap characters cal him Sans.Underswap Sans is very cheerful and energized,unlike his brother,Papyrus who is laid back and cool with everyone.Papyrus is overprotective for Sans and is always with him.

One day, Sans invited everyone to his and Papyrus's house for some tacos.They agreed and came.They are in the living room talking while Sans is in the kitchen preparing the tacos.

(You're Blue.)