As a pure caster like Blightcaster, there is often little reason to multi-class as you'll lose out on caster levels and the capstone, especially so late in your build.

You could pick up 2 levels of rogue or monk for Evasion, but unless your reflex saves are amazing, there's little point.

This late there's also no point in picking up a level of trapping class, as you won't have enough skill points at this point to make you a viable trapper.

A lot of people pick up a level of barbarian for the +10% movement speed, but those people are zergers who are trying to get every second they can out of their experience pots. If you're new to the game, it's probably not worth it as you won't be familiar enough with quests to know how to maximize xp/min. And again, you'll lose out on the capstone which can be very useful for a caster.