One way to do this is to put a black video, made by going to File/New/Black Video, on the track above your video and then place your title above the black video. The black video can be trimmed to fit the duration of your title.
It might be easier to put a black rectangle on the title and then your lettering on top of that. You would be able to dissolve in and out easier.
Or use a white rectangle for that matter. Or gray. Or any color you want. It depends on what the rest of your video looks like. You might want to pick a color from your video track to use for your title background.
If you want to do it all in the titler, instead of a rectangle just enable the title background. But using just the titler means the title and underlying video must fade in/out simultaneously, so a little less versatile.
Another way to do this is to keyframe the opacity on the video, but then you don't get the color option.
Just put the title on the same track as the video, overwriting it.