Items that are dirtydark are darksteel and deal dirtydark damage. If a creature is bearing a dirtydark item that gets shattered or destroyed, it is as though they had taken 8d20 dirtydark damage for the purpose of developing black rot. This is instead of the 4d20 dirtydark damage from darksteel. In addition, every day spent bearing a dirtydark item acts as 1 dirtydark for the purpose of develeoping black rot.
Items that are darksteel can block magically powered abilities even if they are UB. If you are wearing darksteel armor, then magically powered attacks made against you are not AV0. Darksteel items are also resistant to all magical effects. If a creature is bearing a dirty dark item that gets shattered or destroyed, it is as though they had taken 4d20 dirtydark damage for the purpose of developing black rot.