Are you going to solid black? or are you just seeing your image get "saturated blacks" (i.e. darker)?
If it's the darkening, I posted on this a long time ago, and no-one else experienced this, so I figured it was perhaps some weirdness with my display.
If it's solid black then it's most likely that your preview settings for the capture app are set to not display during capture or something like that ( in the preferneces ).