Leaving a cell blank in Excel until the data is entered
You can try using IF, OR and ISBLANK like this:
=IF(OR(ISBLANK(B9),ISBLANK(C9)), "", C9-B9)
You could try a conditional formula using the IF function. Microsoft has a whole article about how they work. Your solution would look like this:
=IF((C9-B9)<=0, "", C9-B9)