Big Tree Cannon is dark TRUNKS's signature attack.
In Dragonball Xz. dark TRUNKS has access to all of Trunks's attacks. This includes his signature attack, the Big Tree Cannon. When dark TRUNKS uses it, though, it turns black. It is the same power is the regular Big Tree Cannon, it's just seems stronger because dark TRUNKS is stronger. It is the same as the Big Tree Cannon in every way, it's just black.