if your cell is really blank, it should work.
Are you sure that there isn't something in there, like a space?
Are you selecting just one cell when you apply it, or a whole range?
Note that if you are selecting a whole range (like the whole column), you need to write the cf formula as it applies to the very first cell in your selected range.
So, in that case you would use the formula:
if you still cannot get it to work, please explain your exact steps.