The wording on Darkness Energy from the EX Ruby & Sapphire expansion says: "If the Pokémon Darkness Energy is attached to attacks, the attack does 10 more damage to the Active Pokémon". Does this mean that you get to add +10 even if an attack does no damage at all to the Defending Pokémon (i.e. EX Sandstorm Murkrow's "Surprise" attack)?

No, it still applies only if there was initial damage to add to. So in other words, you still have to do some damage in order to have "10 more" done.

Relevant cards