Basically it's used like lightmapping, here's how it works:
type into your cfg:
texture 0 textures/default.png (defines the parent texture)
texlayer 1

The "texlayer 1" sets the texture that will be blendmapped. To initiate blendmapping, apply the new texture to a surface, then press 2 on your number pad (make sure you have numlock off), then just click away!
0 turns off blendmapping, the mouse scroller changes the kind of brush you're using for blendmapping.

Hope this helps!
До свидания и заткнись
-P

// 1. Select the texture which you want to blend.
// 2. Hit F9 and read the blue text on the screen.
// 3. Select the surface with another texture which you want to blend with the first texture.
// 4. Execute "/vlayer number_of_selected_texture_which_you've_read_in_blue"
// 5. Hit "0" on the numpad to set blendpaintmode to on.
// 6. Hit "9" or "8" on the numpad to select the desired brudsh.
// 7. Hit "1", "2", "3", "4", "5" or "6" for the blendmode.
// 8. Just hold on your right mouse button while moving your crosshair over the selected area.
// 9. Hit "0" to end blendpaintmode.

sweet thanks for the help guys

/vlayer is easier to use, but you can't use negative slot numbers. Just select a texture and type /vlayer to blend another texture on it (virtual slots can be used). Use F9 while pointing at a texture to know its slot number.