A black bar is a claim: this material is exempt. Like any claim, it has to be supported. The exemption log is the support — record by record, redaction by redaction, with the statutory basis stated.
Segregability is the part most often skipped
Illinois FOIA requires public bodies to release all reasonably segregable non-exempt portions of a record. A whole page blacked out where only a name is exempt is not compliance, and without a log there is no way to tell which happened.
The documented example
Lake County’s 805-page production came back with apparent solid-black masking on fifty pages and no exemption log. See the redaction audit.