Não sabemos ao certo.
A teoria mais popular por aí é que suas chamas são apenas uma versão mais quente das de Drogons e Rhaegal.
Motherboard has a compelling theory that Viserion is breathing a compound called dicyanoacetylene, which would burn hotter (and bluer) than the flames he spewed as a living dragon. The hotter flames might also cause the Wall to vaporize, which is what happened — regular flames would have melted it, resulting in large quantities of meltwater. If all that's true, it means Viserion is now more dangerous than Drogon and Rhaegal, who are presumably breathing regular old fire.
Viserion's Newly Blue Flames, Explained - Cosmopolitan
É duvidoso que o fogo dele seja qualquer tipo do fogo de um dragão de gelo.
"Of all the queer and fabulous denizens of the Shivering Sea, however, the greatest are the ice dragons. These colossal beasts, many times larger than the dragons of Valyria, are said to be made of living ice, with eyes of pale blue crystal and vast translucent wings through which the moon and stars can be glimpsed as they wheel across the sky. Whereas common dragons (if any dragon can truly be said to be common) breathe flame, ice dragons supposedly breathe cold, a chill so terrible that it can freeze a man solid in half a heartbeat."
A World of Ice and Fire
Viserion não conhece nenhuma dessas qualificações além de olhos azuis, o que nos leva a acreditar que ele ainda é um dragão, apenas morto.
Game of Thrones has never been limited by what is and isn't possible in the real world. Dragons, zombies, and shadow assassin babies all populate the world of Westeros, meaning that Viserion could breath whatever George R. R. Martin wanted Viserion to breath. However, it certainly didn't look like Viserion was breathing any kind of "ice beam" or anything type of cold breath that would imply that Viserion is an ice dragon. Viserion is still a fire-breathing Dragon, although a dead one. However, that doesn't mean his blue fire shouldn't be feared. If anything, his blue breath proves that Viserion may now be more powerful than Drogon or Rhaegal.
How Does Viserion's Blue Fire Work? The 'Game of Thrones' Finale Just Changed The Game Entirely - Bustle
Portanto, a explicação mais provável é que o fogo dele é muito mais quente que o de Drogon e Rhaegal.