From what I've noticed blades increase the whole damage effect over the initial and 3 rounds. Traps only increase each hit the DoT takes. Meaning if you have a trap and hit with elf then the initial hit is effected or boosted. All hits each round after are the same as normal unless you place another trap there before each round hit.
So, say you want to boost each hit including the initial of the fire elf. You would need 4 separate fire traps on them for the actual 4 hits it does. Just as if they put a ward on during one of the rounds while under the DoT, it only effects that round hit.
Hope this makes sense. I very rarely use traps for my DoTs. I always use a blade trap before it i cast the spell so the overall damage is increased.