On what type of champions is it best to buy a blue trinket?

When enemy team has enough tools to make picks on you and your teammates for daring to get in range to use a Normal Ward.

Mostly the Mid and ADC players get this, Support and jungle red trinket (for obvious reasons) while top sits on Yellow trinket.

vs assassins

You have a semi-global ult.

Global ults. If you need to be safe if you need to ward something like drake oe baron.ect...

Champions with long range ults, and those who don't want to get anywhere near the bush to ward it since they're probably going to die if they do.

So...Basically you want it on most ADCs.

Level 9 :)

ADC's and Mid laners.

Top, Jungle and Supports will usually buy Sweeper.

Most ADCs.

Pretty much anyone with a really long range skill like Nocturne.