Which effect is this?

I would call this a semi-transparent overlay to improve the readability of the text.

Steps to recreate this effect:

1. Create a Solid Black color matte
2. Put it on a video track above your footage
3. Drop the opacity of the Color Matte
4. Create a cut-out using the mask tool, or using a 3rd party Crop Effect to create a diagonal.
5. Finally, add a video transition to reveal and conceal the black color matte (e.g., Wipe or Push.) 

I personally recommend FI: Wipe Impact, or FI: Push Impacts, by Film Impact because of their easing controls and motion blur. 

It may be an animation created in After Effects and placed on a track above the main video.