I am experiencing similar thing. It might be the usdz_converter itself is still kind of buggy? Some OBJ just does not convert to proper USDZ with texture.
I did manage to export OBJ that can have the texture albedo somewhat, when I am using Blender Internal, assigning material and texture slot then export the OBJ from this BLEND.
How about you try outputting the USDA using usdz_converter, then manually modify the USDA linking the shader + texture mapping files from this USDA, before converting the USDA to USDZ.