I had this problem too. It you make sure all of your maps and textures are also on the slave machine in the same directory it should fix it.
Or create a local folder called C:\vray and put all of your materials in it. Then create a local mapped drive letter Z:\ that points to C:\Vray then on all of your nodes create a Z:\ that points to the servers c:\vray as well and that will generally solve all black block issues. This way the local 3dm is looking for materials in Z:\ and so are all of the nodes.
I have a problem with slightly patchy depth of field when distributed rendering is on. Has anyone else found this? Any news on a fix?
Maybe that is what you ment by this: