You have layers and can create a layer, drag it beneath your active layer and put the water mark there, or create a layer above and put it there and make it transparent if you wish.
The interactive blend tool will show the forbidden symbol until it senses something that it blendable underneath it. There are tutorials on the web as to how to use blends that will show you more about this. You must have two items to blend between or blends will not work. The blend works by dragging the mouse once the forbidden symbols disappears to the other object at the opposite end of the blend.
Every day's a new day, "draw" on what you've learned.
Sally M. Bode