From my experience since the patch that changed it - blizzard only proces on the initial cast ignoring if it hits anything (you can spam lvl 1 blizz on the ground and you should get 1 CC per 10 casts). On the other hand other AoE spells imo work differently - the 10% chance on CC is counted from every unit it hits.
For example if I go versus pack of 10 mobs blizzard has 10% chance of CC, but CoC/Nova/AE... has "10*10%" chance of procing the CC. I know it's not 100% but you see my point. It's been a while since I studied math, but in reality it might be something closer to this: 10 mobs CC%=100*(1-0.9^10)=65.13%
Since I started using Arcane Missiles lately in BWL I have noticed that the CC can proc on every of the damage tick of the missiles -> like blizzard used to. So should that spell be fixed too or what? It might be "abusable" in the way of spamming rank1 Arcane Missiles when I'm OOM just to get the CC buff. And if my math is correct, then the 5 dmg instances of Arcane Missiles have a total of 40.95% to proc the CC. I am too lazy at the moment to calculate the Mana vs DPS efficiency regarding Frostbolt's 10% vs AM 40.95%, but it might not be a complete waste of time.