I understand your confusion as I used a blend mode not the normal blend. That's why the blend 0.5 looks a bit darker than expected.
And yes, your picture shows exactly what I meant. A complete masking of video a without blending. This would be great and extremely useful for every video composer.
Many thanks
Edit: I've corrected the picture title as it wasn't blend 1.0. Instead I changed the blend mode again. Blend 1.0 of course shows "video b" complete. Sorry, was in a hurry.