In Excel 2010, how do you apply CONDITIONAL FORMATTING to blank cells?

What I have done in the past is use this ... if [cell] ="" ... apply the conditional format to the cell (fill the cell with an assigned color).

This particular worksheet is used to track and rotate employees' non work day in a weekly schedule. All employees have rotating days.

It has worked perfectly until the first time i edited the file in Excel 2010.

???