It seems that Owl's Lullaby gives no effects. Even it's non-Moonlight effects are not occurring. None of it's effects working at any hour of the night, in any zone, open or closed.
Another possible suggestion, can it have it's darkness removed at level 8 so it could be cast within safe areas? Unless a toggle for 'safe area casting' was allowed, for any other effects that currently exist or down the line, to allow for such.
Maybe just make the darkness the vfx, not the actual spell.
Update on this one, it appears that bards are able to trigger the darkness even during the daylight hours with this song.