As the delay is changed from 0 to 24 (inclusive) (but this should not include 24) one transitions from all 0's to all 1's (black).
The modules are configured to return clock/2 and the BOC is set into a mode where the samples are taken at half frequency. This means the transition may be from all 1's to all 0's.
The delay is then set to a point far from any edges.
Example plot... 
An example of one of the BocSetupTests