Actually, it sounds like the same lumens are being produced, the LCD is just putting up more black, which is blocking more lumens from being visible. If that is the case, it will not use any less battery.
It probably uses slightly more really. There's the CPU/GPU usage associated with the filter, and the trait of LCDs to use the most power showing black.
Here's a quick explanation. "Brightness" normally controls the LCD backlight, which is generally either a lamp (more common with laptop sized screens) or an electolumenescent material (glows when ~1000 volts AC is passed through it, the transformer for such sometimes creates near-ultrasonic noise that young folk can hear if the engineers aren't careful with the design). This light passes through the liquid crystal grid. Liquid crystals are polarizing light filters that have the interesting property of changing the direction of polarization when an electric current is passed through them. Next is a color filter (red, green, blue), then another polarizing filter (normal type that doesn't change orientation).
The backlight is one unit, so it's on to whatever power your settings dictate. To show full brightness red, green, or blue (depending on the color filter) the liquid crystal's polarization orientation matches that of the second filter, so no light is filtered out. To show black, a current is passed through the liquid crystal, which shifts the polarization 90 degrees so it filters out nearly all of the light from the backlight. Darker colors are achieved by not quite rotating the polarization all the way. Technically, one could make an LCD that used the least amount of power when showing black, but that'd use more power overall (webpages & documents tend to be white), and it'd look weird when it was off.
Of course, OLEDs work completely differently. They have a tiny LED for each subpixel (generally four per pixel, which is also different), so black is the most energy efficient color for them.
TLDR: Brightness controls the backlight, LCDs just use power to block that light if you want black.