Blue Slimes are a highly common variety of slime. They appear in Forests during the day, or at any time in the Underground layer. Blue Slimes will usually not target the player, but will become aggressive when damaged, during the night, or at any time when underground.
Like most other slimes, Blue Slimes will be passive towards any player with the Royal Gelaccessory equipped.
Like Green Slimes, Blue Slimes seem to have a confusing behavior, bouncing away from the player sometimes. This is common in multiplayer.