Transitions cause transparency until their duration gets changed
If you create a transition between two clips, and use a clock wip or linear wipe or anything more obvious than a crossfade/dissolve, you will notice that the two clips become less bright as they are partly transparent with each other for the duration of the transition.
In the example below you can see the LEGO pyramid through the "" linear wipe that comes from the top-right corner, and the image is very dark overall:
This happens at the beginning when you switch from a crossfade transition to another type of transition. However, when you move one of the clips, effectively changing the duration of the transition, the transparency side-effect disappears.