How might I implement a "subtraction" blend mode?

[Pixel_Outlaw]

While looking through the SFML tutorials, I've noticed that it offers some blending modes. I was wondering if it is possible to create a blending mode that subtracts the values of the source from the destination? Is there a way to do this for a Sprite? I know an entire shader can be written but is there a way to do such a thing for just one Sprite object? I'm trying to create the effect of transparent material layers that darken. Something like how stacking colored plastic sunglasses lenses gives a darker result.  

[Laurent]

There's no such a blending mode in SFML, but you can get the same result by drawing a semi-transparent black sprite -- the more opaque, the darker.

[Pixel_Outlaw]

Thanks Laurent.

I just wanted to make sure that I wasn't missing something.