The black dot is simply an indicator marking those threads in which you have posted a reply. It's a visual reference for you to identify where your posts are so you can differentiate at a glance threads you've posted in versus all the others.
It's not a seal on the envelope. The envelope is closed only if there's no more posts in the thread since the last time you were on The DIS. If new posts are placed there, the envelope will be opened, but your black dot will remain.
Hope this helps.