XL's format convention has 4 sections separated by semicolons. By
default these sections are for positive numbers, negative numbers,
zero, and text. To get what you want for zeros, choose
Format/Cells/Number/Custom General;General;-;@
You can't format a blank cell to show a value (or then it isn't
blank). Instead, enter a hyphen (or zero) in the cells you don't
want another value in.