Connect the BL line to +5V and the backling will be fully off.
Leaving it unconnected or at 0V will give full backlight brightness, you cannot increase the brightness unless you hack the board and increase the LED current. Increasing the LED current signficantly may damage the LEDs.
Connecting BL to 3.3V may not fully turn off the backlight.
A PWM on BL will give variable brightness, 1KHz will be fine.
These displays give quite poor readability in sunlight as the backlight is not very bright compared to the glare frome the sun. You will need to keep the display shaded as much as possible to see it clearly.