What does the black mark in the pic represent? 

See "F": http://voxcommando.com/mediawiki/index.php?title=How_to_use_VoxCommando#Understanding_the_main_window_interface

The black line was James's attempt to make it clearer to people that audio input levels higher than that are bad, because it seems that people weren't getting the message.

The line may be removed again in future versions, though.

It should be noted that this is just a guideline.  Values a bit higher or lower than this should be OK for your peak volume.  It will depend on a few things.

@james
Maybe adding your comment as a tooltip when hovering with mouse would help users to know what it is about