Concealment

Concealment in DDO refers to Concealment Miss Chance

Concealment Miss Chance gives the subject of a successful attack a % chance that the attacker missed because of the concealment. If the attacker hits, the defender must make a miss chance percentile roll to avoid being struck. Multiple concealment bonuses of the same type do not stack, and currently almost every single one is considered the same type - enhancement.

Concealment from external sources, such as from a cloudkill also won't directly stack with concealment from buffs like blur. But since it is considered a 2nd source of concealment, it will force an additional percentile roll, so the monster may have to roll a d100, and get over 20, twice in a row to successfully attack you if you had both blur and he was subject to a cloudkill.