Rendering Black Tiles
Rendering black tiles is usually a symptom of running out of memory.
If you are rendering inside XSI interactively and using satellite it may indicate that the render of a tile on one of the slave nodes is running out of memory. On a renderfarm farm using XSIBatch it may be because the tile size is too high.
By default mental ray decides the tile size and this may not be optimal for your scene. In the Render Optimization settings try setting a lower tile size such as 16x16.
Small black tiles or pixels means something else, this is likely a problem with a Shader.