I struggled with that issue too, based on the same tutorial.
The solution is simply to follow instructions described in the tooltip of the effect material property of your retainerbox :
- Setup your effect material blendmode as AlphaComposite :
This blendmode will get rid of the black bakground but require some additionnal consideration about the final color (see next step)
- Make sure to multiply the Texture parameter color AND alpha by the desired final alpha :
