why this face is always black? I unwrapped the whole mesh, but one face is always black, I tried CtrlN and ShiftCtrlN but there is no change.

in the 2d editor "View > UV Local View" isn't selected

you can get the blend here : http://www.pasteall.org/blend/37300
The black face's normal is flipped so you have to recalculate the normals (Toggle into Edit Mode Tab > Select All A > Recalculate Normals Ctrl+N)
and also move the light source to see that the texture is there and it's just bad lighting causing the black face.