Hey all, so I just wanted to post a hint on how to solve this issue if it happens to you as it did to me.
If your model shows up completely black inside the model viewer or hammer it means that you forgot to re-export your VTFs. It used to be that it would use the black and pink texture for this but apparently in CS:GO at least, this isn't the case anymore.
So simple solution to a simple problem.
Thanks for sharing this info.
Another way it can sometimes happen is if you are using a WW Material and the shader type is incorrect (for example, using LightMappedGeneric on a model).
In this case too, changing it in Max is not enough. The material must also be re-exported.