|I'm a little confused by your mixture of Formatting, making a number negative and the need for a formula.|
What exactly are you doing with the formula? Actual examples of input and output work best, not just a "word description".
I'm thinking that a Custom Format might get you where you need to go, but I can't offer any suggestions until I know what you are trying to do.
In general, you can't "skip a blank cell" with a formula, but you can have the formula return nothing if the cell is blank.
For example, both of these formulas will return nothing ("") if A1 is empty, otherwise it will multiply the contents by 2:
=IF(A1="", "", A1*2)
=IF(A1<>"", A1*2, "")
They didn't skip A1 if it was empty, they returned the specified value, which in this case was "".