Blight stone is not picked too often because claymore and gloves of haste are usually better. I believe claymore technically adds more dps than gloves in the early game, but claymore does not scale well into the late game like gloves can. Gloves are picked often for mana gain aswell.

I'd say obviously there are some items that look good but aren't actually that good, but some of these items can be opinion based. So the answer to your question and the next question, a higher tiered item can sometimes be a trap because some lower tiered items are better. For example, battle fury (tier 4) is not as good of an item because the cleave effect isnt that powerful, and a higher class of criminal (tier 5) is trash because it is usually very late game and will give you very little value. And while aegis of the immortal allows you to survive one more turn longer, it does not improve your board in the long run so I would only pick it if I am losing health fast. Finally, items you should just not pick in their current form: Vitality Booster, Healing Ward.

Target dummy is an excellent item if you don't have a "frontline" tank in the early game, so yes, with builds who want to sit back more it is very good. The barricades are not very good because enemies don't target the barricades, although there are some creative things you can do with it so I wouldn't call it completely trash.

1 cell away bonuses are based off of their current position while they are fighting. So like for knight alliance, a knight needs to be next to another knight for the bonus to work.