Well I went ahead and made a rough demo. hopefully it has the Style of Unlock-able Object you need.
If it isn't then explain a bit more on your situation, and i'll be able to create a demo with much more value to you.
Explanation of The Posted Demo...
I simply created an actor similar to the one covering, but I used the Gray Color Scale.
I Took The Gray Copy And Made it As close to 50% Transparency as i could.
After That It's just a simple destroy Cover Actor When my arrow collides with Brick. and Create Cover Actor When Arrow is Collide Free of Brick.
I Hope This Info Was Useful to you,
Dellzby