I didn't know what the black dot was either until I wrote the thread about the Disboards messing up my computer.
It's a dot on the envelope that reminds you that you have visited that thread before. ( I think).
I don't see what's so great about it, but some people seem to love it!
Apparently, there are quite a few of us having problems with our computers when we get on the Disboards and Alex thought it might be caused by the black dot.
The dot is gone but I'm still having problems.
Happy Valentine's Day!