I used darkening 0.2 yesterday while spectating a game. It works awesomely!
Many thanks for this command!
EDIT: prospective users should note that higher numbers darken more.
/luaui darkening 0.2 - decreases brightness by 20%
0.5 halves brightness
EDIT2: Note that this also applies to the L key, so you don't want to darken too far or you won't see your los