A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
I actually cannot reproduce the error using backspace as it responds the same way as delete in my quick test. However, you might try adding ="" to your validation list as it is an empty string which is not quite the same as a blank cell.
Edit: after some additional testing in an attempt to reproduce the backspace behavior, I would suggest that you leave the Ignore blank option checked in the Data Validation definition.