Overinking misprints are the result of a malfunction during the printing process that results in too much black ink being applied to the same area. These misprints often affect the black treasury seal. The more ink that has been applied incorrectly, the more the error will be worth.
Unfortunately for collectors, overinking misprints are easy to fake. Because of fake errors, the value of all errors goes down. Extra blank ink misprints can be as mild as an extra drop of ink to something as major as the entire face being covered in black ink.