All configurations are default, I have not changed anything, in the old version is correct, under is my screenshot:
This is a screenshot of Blender 2.8:
And this is a screenshot of Blender 2.79:
When I open the new project (created by Blender 2.8) in Blender 2.79, the video is dimmed too.
Adjust->Compositing
. Check the blend mode and opacity value. – Marcosmarcotte