That should be done in a graphics editing program prior to adding it into your project. However you can try setting the Blend Mode in the properties to Additive. Generally that's used for animations like explosions with black backgrounds to blend them into the game so you don't see the that it's actually a square graphic against the background. Anytime you the additive settings on multiple animations, remember to keep them all on the same layer to keep it from effecting game performance. And there will a slight degree of transparency created to the graphic.