Hi -
Okay in your Project, make a new Empty Level (the one with Black nothingness) Add the Particle System to the new empty level and while the emitter is selected press the “?” key on your keyboard. This will fire off the effect, does it look like it is still not fully translucent.
If it does please take a screenshot and let me see what it looks like with nothing in the background. Chances are though that it looks like it does in the Particle System viewport. If that is the case, then you will need to either adjust the Material to not be Additive but straight Translucent, adjust the color and roughness of the level underneath the effector adjust the Post Process Settings in your level. The Additive behavior is causing what appears to be a not transparent effect.
Thank You
Eric Ketchum