there is no function for brightness in the python module.
Please take a look at a simple script I´ve wrote for that purpose:
from blinkstick import blinkstick
led = blinkstick.find_first()
Brightness = 20
r = 0
g = 0
b = 0
r,g,b = led._hex_to_rgb("#ff0000")
r = (Brightness / 100.0 * r)
g = (Brightness / 100.0 * g)
b = (Brightness / 100.0 * b)
led.set_color(red=r, green=g, blue=b)
"Brightness" is a value between 0 and 100. Feel free to try it out.