Ruling Black Belt and Damage-less attacks

[Charganium the Bold]

If I had a Pokémon with an attack that did 0 damage, such as Mime Jr., and I played Black Belt, would I do 40 damage or 0?

I'm asking because this happened on TCG Online.

 

[DNA]

You would do 0.

In order to do "more damage", you have to be doing damage to begin with.