Kirk has found a bug in iTunes 12 whereby selecting a batch of tracks and deleting their Genre tag via the Get Info panel inserts 8 spaces instead of empty text into each track's Genre tag. This creates a Blank Genre that is actually displayed and selectable in Genre lists.
This AppleScript will correctly delete the Genre tag of each track in a selection:
tell application "iTunes"
set sel to selection
repeat with thisTrack in sel
tell thisTrack to set its genre to ""
Also, any number of other scripts that can edit the Genre tag, like Multi-Item Edit, will get the job done.