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.
I had a similar issue, and I was able to get it working with textures. Here is the command I used.
xcrun usdz_converter edieThepig3.OBJ EdieThePig.usdz -g piggy -color_map pigLowPoly_defaultMat_AlbedoTransparency.png -metallic_map pigLowPoly_defaultMat_Metallic.png -roughness_map pigLowPoly_defaultMat_Roughness.png -normal_map pigLowPoly_defaultMat_Normal.png
the '-g piggy' is very important. it's the group name that the textures apply to. In my case that was the name of my root node exported out of Maya. If that's wrong, or missing, or includes unsupported characters, the textures won't apply.
Here is my final usdz asset: http://www.natwales.com/demos/usdz/
im dealing with a similar issue, it creats the USDZ. In the thumbnail it seems to have texture and color but when i click to load it, it wont load on my ios phone.
Thumbnails in text: https://s8.postimg.cc/r7qej1i5x/IMG_3318.jpg
Error loading: https://s8.postimg.cc/4vslpvo9h/IMG_3319.png
Itll load when it has no texture (looks grey) but the minute i add color, it wont load on my phone.
Here is the code im using:
xcrun usdz_converter Performance_Tech_Motorsport_Oreca07.obj PerfTech2.usdz -color_map v6Performance_Tech_Motorsport_Basecolor.jpg -metallic_map Performance_Tech_Motorsport_Metallic.jpg -normal_map Performance_Tech_Motorsport_Normal.jpg -emissive_map 01Performance_Tech_Motorsport_Emissive.jpg -ao_map Performance_Tech_Motorsport_AO.jpg
Make sure the size of images is not too large, I keep it 2048 x 2048.
Unfortunately at the moment, when USDZ crashes on iOS, there seems to be no crash report -- I might be wrong.