blanking edges between two formats (16:9 + 4:3)

It sounds like you're describing "Pillar Box".

1. Create a Color Matte (File > New > Color Matte...) with the desired Gray value (#808080 or 128, 128, 128 is 50% gray).
2. Add it to a Video Track that's in front of all of your Clip Boundaries in the Timeline and extend it over your footage.
3. Add a Barn Doors Wipe (Effects > Video Transitions > Wipes > Barn Doors) to the Color Matte in the Timeline.
4. In the Timeline, extend the Barn Doors boundary it to the length of the Color Matte boundary.
5. While the Barn Doors boundary is still selected in the Timeline, go the Effect Controls for the Barn Doors and set the following: Start to 25%, End to 25%, with Reverse enabled.

It should look like this:

It's probably worth noting that with the Transition set to the same duration as the Color Matte, it will be difficult to click and drag the Color Matte later.

Have you considered matching what's done professionally for picture that doesn't fill the frame?  You'll notice in documentaries and news broadcasting that the picture is scaled to fit the width or the height of the frame and then the resulting pillars or letter boxing is filled with the image scaled up and blurred (behind the picture being shown).

It would then look like this:

(The dog in the photo is Buddy.)

-Warren