bluebox effect

How can I use a bluebox effect. I try to add this effect form effect tree and I added second clip (As a background). But without effects.
I try select a color also with no results.
Can somebody tell me how to do it?

1. Add blue box or color selection to top track
2. Add a composite transition between the tracks

Step 1 creates an alpha mask.
Step 2 uses that alpha mask to switch between tracks by making the top image locally transparent.