1) Open your model in war3model editor
2) Go to: Windows -> Material Manager
3) Now click back into the main window of your model editor (so you can turn the model, etc.)
4) Hold down left "CTRL" button and left-click the blackened part of the model (make sure you don't accidentally click the glow around his weapon, etc.)
5) Now that part of model should be highlighted in your Material Manager window - double click the highlighted material
6) New window called "Materials" opens, there is a window inside it called "Layers", in which usually 1 or more "MaterialLayers" are
7) Open one of the MaterialLayer by doubleclicking it. A new window called "MaterialLayer" opens.
8) In the upper-right part of the window is a "Texture ID" field (Don't mistake it with "Animated Texture ID"). Check the scrolling menu for its texture - if it's "Replaceable ID1 or 2", then leave that (it's the player color), if it's (none), change it for the model's texture - in case of Arthas model, choose the "Textures/Arthas.blp" texture.
9) Click "Ok" until your are back to Material Manager window
10) Check if your problem has been solved.