From the Super Mario Wiki
Blue Blazes are blue normal ghosts encountered in Luigi's Mansion. They are Blue Twirlers with water protecting their thirty-HP hearts. They are normally found only in the Safari Room, in the east wing of the third floor of the mansion, although one is encountered in the Tea Room during the blackout. The Blue Blaze has a massive earthquake attack that costs Luigi ten HP, as with its non-elemental counterpart. Ice is the key to a Blue Blaze's heart. Being weakened by ice, it completes the triangular Elemental Medal relationship also found with the Temper Terror and Flash. In the Safari Room, the Blue Blazes appear when Luigi shakes the middle deer head, along with two Garbage Can Ghosts. In the Tea Room during the Blackout, a Blue Blaze appears in the center of the room.
In the beta, there were going to be three Blue Blazes playing poker; though the formation itself wasn't in the final game, they made a short cameo when Mario was being turned back in the ending.