So... yes, I've experimented with that on the Mac too (I think it was "Shady" or something?), so you certainly can.
But that doesn't reduce contrast beyond what's inherent in reducing brightness -- the blacks when the backlight is at minimum power in a dark room are black. You seriously don't get blacker than that in any practical sense that is relevant to the human eye, certainly not on the high-quality displays MacBooks have.
I don't know what you are trying to say with the word "contrast", but a utility like Shady does exactly what you're asking for -- reduces brightness below the minimum -- with zero negative optical side effects. It seems that whatever you're expecting it to do (dimmer but better contrast) is literally a physical impossibility.
But that doesn't reduce contrast beyond what's inherent in reducing brightness -- the blacks when the backlight is at minimum power in a dark room are black. You seriously don't get blacker than that in any practical sense that is relevant to the human eye, certainly not on the high-quality displays MacBooks have.
I don't know what you are trying to say with the word "contrast", but a utility like Shady does exactly what you're asking for -- reduces brightness below the minimum -- with zero negative optical side effects. It seems that whatever you're expecting it to do (dimmer but better contrast) is literally a physical impossibility.