As Dawny33 speculated, and speculated correctly.
It was fire.
This is confirmed in an interview by The Huffington Post with director of the episode " Dragon and the Wolf ",
Jeremy Podeswa, when asking the all important question "Was it Ice or Fire?" .
(Emphasis mine throughout)
“The way I looked at it was, when the sept burned down, that was green fire, and so then the dragon is going to have some kind of blueish fire ,” he said.
“ It’s certainly still fire — it has the ability to burn the Wall and melt snow.
But it’s going to have a different kind of magical quality to it, because it’s coming from an undead dragon.”
The Huffington Post in an interview with Jeremy Podeswa This is re-confirmed in an interview with IndieWire where he discusses the humour of the acceptance of Green fire but not blue fire, although that 'chemically' the two would be different, coming from a dead being.
“It’s funny because people really accepted the green fire [of wildfire], which burned down the Sept [of Baelor] and the sea battle in the earlier season,” said Podeswa.
“I think that fire can have many different manifestations.
I think that the ice dragon fire is clearly still fire because it does melt The Wall and tear it down, but it had a particular quality because it’s coming from an undead dragon.
So what the chemical nature of that is and the chemical nature of fire would actually be different coming from a non-sentient being.
So there’s
definitely some thought into that of what that should actually look like.
But obviously, an ice dragon is going to have an icy kind of fire
but it’s still fire.”
Jeremy Podeswa breaks down the final episode for IndieWire Finally, when talking about the amount of practical vs visual effects used in the episode with The Hollywood Reporter , Jeremy Podeswa reveals that there are interactive elements with the scenes where the "flame" tears the wall down. "...
And there are many interactive elements also that people might not be aware of, like the dragon flame and the impact of that and what it does in terms of the effect it has on the Wall..."
The Hollywood Reporter in an interview with Jeremy Podeswa
In addition, the official script to Season 7 Episode 7 "The Dragon and the Wolf" states that it was indeed blue fire : Hovering near the base of the Wall, the ice dragon shoots forth a steady stream of blue fire, thick as a tree trunk, strafing back and forth along the base of the Wall.
"The Dragon and the Wolf", page 62 - Official Script
scifi.stackexchange.com/users/68872/Edlothiad
It is fire
The thing which the dragon was breathing radiates like flames, which only fire can do.
Never seen any creature in Scifi breathing ice which radiates flames Also, one can clearly see black and grey smoke coming out of the wall wherever the blue fire touches it.
Ice on ice creates white smoke.
But here, it is more greyish towards the upper side of the smoke, which again supports the view that it is fire.
Viserion is known to be smaller than his other two brothers, Drogon and Rhaegal.
So, unless and until he magically grew very strong under the aegis of the Night King, there is no way he could have breathed ice so strong which can bring down another ice structure like the Ice Wall [which stood strong for 8000+ years].
So, it should be fire.
To be warned that all the answers here would be speculation.
We shall only (probably) know in the next season.
scifi.stackexchange.com/users/53012/Dawny33
As the other two answers say it was fire, I will take the other position: it wasn't fire.
When the undead dragon breathes on the encampment at the top of the wall, we see no items catching fire:
Every time we've seen the living dragons breathe, things have caught fire.
Furthermore, the damage to the wall is inconsistent with fire damage to ice; we do not see melting, we do not see smoothed surfaces where the "fire" has hit.
We see particles which could be steam -- or they could be dust, particles of the wall, etc.
We do see jagged edges and flying debris consistent with physical impact.
This is inconsistent with the fire of the living dragons, which has been shown to hit humans without explosive force, simply lighting them on fire: Given a lack of signs consistent with fire damage, we are left with what we do know: it is some variety of force-causing emission which acts differently than the fire breathed by living dragons.
It may be ice, or it may be something different entirely.
scifi.stackexchange.com/users/None/
It is some kind of magic fire, it was definitely fire as
it brought down a huge portion of the wall which was made of ice and generally
, fire melts ice!
(Though the impact was different than from the previous episode when Dragons were vanquishing the Wight walkers and ice)
In chemistry, we do have substances which burn with a blue flame.
We need to wait for the dragons to fight each other to know which flame is stronger.
scifi.stackexchange.com/users/29106/ABcDexter
I'm certain it's magical dragon fire...
White walkers have blue eyes and the dead they reanimate have blue eyes too.
Also, those magic sphere bombs that Leaf had were blue.
I'm guessing this type of 'un-dead' magic has a blue quality.
scifi.stackexchange.com/users/89253/Kaur
Major spoilers for the Season 7 finale of  Game of Thrones .    Did Viserion (Night King's dragon) breathe ice or fire to bring the wall down?   Reasons for Fire:    - It  is  a dragon...  - It  looks  like fire, just blue...    Reasons for Ice:    - The Night King's magic is ice based.  - The Wall does not appear to be melting, but exploding
Was it Ice or Fire?
scifi.stackexchange.com
<game-of-thrones><dragons>
scifi.stackexchange.com/questions/168185
scifi.stackexchange.com/users/57310/Skooba
Any answer will be speculation, but it is generally agreed that Viserion breathes fire to bring the wall down, even though it has a magical quality because it's coing from an undead dragon. However, some people may dispute this and claim it to be a magic form of ice. Views vary but the most authoritative statement comes from director Jeremy Podeswa who says it is fire, but the fire has a different magical quality because it is coming from an undead dragon.
According to director Jeremy Podeswa it is fire, but the fire has a different magical quality because it is coming from an undead dragon. It must be fire because ice on ice creates white smoke whereas this fire creates grey smoke. It wasn't fire - it must have been a jet of particles like ice or dust. There are substances in chemistry that burn with a blue flame - and the blue colour is like the blue eyes of the white walkers and the reanimated undead.