2D Mobile: How to mask a gameObject with another gameObject (with soft edges)

Hello, we're developing a 2D mobile game which has no light system.

What we want to achieve is:

Dark areas in certain levels which consist of dark sprites, drawn in photoshop, like this:

Our player has a boomerang which he will be able to throw. The boomerang has a light aura which is invisible in the light, but iluminates within a certain radius when in the darkness.

Finally, here's an image of when the player is inside the dark area:

So, does anyone have any idea on how to achieve this? What we've been trying is having 2 different gameobjects (one for the aura and one for a dark area) and try to blend them using shaders. The problem is that we are not experienced with shaders at all. If anyone could come up with a solution we would be most apprecciated. Thank you for reading!