I received a W-2c. If Box 2, "Federal income tax withheld" is blank for both "Previously reported" and "Correct information," do I use the amount originally reported?

The W-2c only reports changes from the original W2.  If any boxes are blank, the amounts did not change from the original W2, so use the original amounts.  Where they are not blank, substitute the old amount with the newly reported amount.