Excel Cell is blank but not empty

All of my cells have formulas linking to other cells and sheets within a spreadsheet I have created, therefore none of my cells are 'empty'.

I have researched extensively but I cannot work out a formula to have cells be blank when other cells are blank (but not empty as they have a formula). Any advice, I have tried the following:

• =IF((C15<0)," ",($B15*$C15))
• =IF(ISNUMBER(SEARCH("%",C15))," ",($B15*$C15))
• =IF(ISBLANK(C15)," ", ($B15*$C15))

screenshot of my spreadsheet