The default missing texture in DX10 is different to DX9.
In DX10 a light blue texture is used whereas in DX9 its black. This is a problem chiefly because add on scenery is normally tested against DX9 and the failure to include a relatively inconspicuous dark texture can be missed as the default dark DX9 texture does not stand out. In DX10 the light blue stands out very much more and is a distraction- this is even more true at night.
I found the texture is in the main FSX directory (i.e where fsx.exe lives) and is called bkg.bmp.
I created a black 24 bit bit map (rgb values all 20/255) and swapped it. Missing textures now look similar to DX9.