I haven't used the behavior before, but I do fades like this normally:
If Object is Overlapping | Set Opacity of Object to Object.Opacity - 1000*dt
If Object is Not Overlapping | Set Opacity of Object to Object.Opacity + 1000*dt
The first line says that if the object overlaps, keep subtracting from its opacity (fade out). The second line does the opposite. It keeps adding to its opacity (fade in). Replace 1000 with the speed of your fade.