Sounds like you're choosing Text with a black background - this can be made transparent with a slider in Properties, but easier still is to choose Text with the chequerboard background, meaning it's already transparent.
Just tried, but couldn't reproduce your symptoms - you have the title in a higher track than the video, and immediately above it?
(edited addendum ...)
Just found I could reproduce the background fading to black if I inserted a keyframe and changed the background from transparent (bottom of slider) to black (top of slider) - this may not be what's causing your situation, but check that you haven't accidentally inserted such a keyframe.