Technically there is no different between windows and blanks.
The windows make a cheaper conversion, however they are more of a maintenance problem as they have to be inspected and sometimes replaced.
Ya pay yer money and take yer choice.
The freighters with blanks normally a couple of places each side with windows to look at wing leading edge and engine intake for ice secretion plus ability to look at landing gear mechanical indicators ( where these are on the top of the wing.)