Black level compensation in multi projector setups

Hello
I have a K5000 and I made a little program for setting up warping and blending. It works nearly perfect.
But I have a question about the black level compensation (offset texture). I set a texture so that a black image shows ok on the screen. But then a gray image does not look ok. And if I set another texture, so that a gray image looks good, the black image does not. Do you know what I mean? Do you know, what I’m making wrong?

Thank you very much.
Greetings, Marco