Are -dmg modifiers applied to barrier?

If not, should they be?

Also wondering if poison affects barrier.

Barrier simply temporarily increases your vitality, so no they don't get affected by your dmg modifiers (vitality isn't a factor in those) - it only absorb some of the damage that comes through after the calculations are done.

Since barrier isn't healing (green numbers) but rather a transform-like upscale of vitality, there is nothing to suggest poison affect it in the least. If it does, I'd imagine barriers would get 33% less of a bonus from your healing power, but it already scales so low it's not going to make a notable difference unless you run minstrel/magi.