sed one liner to Delete blank lines - Help required

Hi,

The above is the content of the file.(blank lines in between lines,at the begining of file and at the end of the file)

I am applying the following sed command

the output is

Note : inorder to represent a blank line i have used 'BlankLine'.
My doubt is how 1 blank line in begining and 1 blank line in between the lines are preserved. Also all blank lines at the end are removed.

Can anyone please explain this.?


Thank You