[C#][DirectDraw] How to obscure an image?

[Fraktalite]

Hi everybody,

I have an image which I would like to display darker. Could it be possible to do this using DirectDraw?

Here is my code to display the picture:
backBuffer.Draw(SpriteDest, spriteSurface, spriteSrc, DrawFlags.DoNotWait);

Have I to add DrawFlags in these func?