Since when did Blizzard have a cast time? I thought it was instant like warlock’s rain of fire.
Nah Blizzard has always baseline had a cast time, or a channel way back in the day.
During frozen orb it’s instant cast due to this talent:
Might be what you’re thinking of.
Like 10 years ago in the major Legion class reworks.
Nah, it didn’t use to have a cast time until Legion.
It was a channel before Legion… so sure not casted, but not instant.
Not sure what it was pre-Cata, I started playing then. But Cata-Legion it was a channel, then a cast since. It’s never been baseline instant cast though, at least since I’ve been playing the game.
That frozen orb talent has been around for a while, and there has been a few patches/seasons where frost had pretty much 100% uptime on orb with CDR. So in practice it was instant cast most of the time with those particular talents. But the spell baseline hasn’t been instant, at least Cata onwards, can’t speak to before then.
Yeah it was a channel before but that’s not what you said initially, you edited your post after I quoted it. I never said it was instant, that was the OP’s inference, I never addressed that and just answered his question about the addition of a cast time. Blizzard has been one of the most frequently changed abilities over the years. Its change log reads like a 19th century Russian novel. 
And, btw, Rain of Fire was also a channel back in the day and hasn’t always been an instant cast. It transitioned to an instant cast back in 2012, MoP era.
Yeah I forgot it used to be channeled when I first made the comment lol.
The point is tho it’s not ever been instant. Ya know what the OP of this post asked lol.