I use GIMP to make the textures.
But I believe that Blender does not like .dds file.
.png should work, but for some reason, yours ended up black.
In Blender I was able to find the texture and save it out as a .tga file.
Then in GIMP, I converted it to .dds
GIMP does not have a .dds conversion by default and one can be downloaded off of the GIMP repository site.
gimp.org should get you the link. It is called Normal map plugin.
This post has the links on them.
If you are using Photoshop, Then search online for the Nvidia normal map and .dds file plugin.
It is recommended that you use .dds file format for textures, but we also support .tga, .jpg, .png.
One of these 3 should allow you to see in blender and then export out in x file with the export textures option set in the export of the x file.
Hope that helps more.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit, Screen resolution 1680 x 1050.