It could go either way as far as models or bsp.
There are a few good tutorials from the WAW days that will guide you through this process.
I noticed treyarch will use blend textures on small terrain patches to get the burnt look on top of bsp brushes. A lot can be achieved through csg subtracting and breaking up the bsp into small chunks to achieve this effect. BUT REMEMBER to always work inside prefabs when using csg subtract, else you will destroy all your zombiemode volumes ect and make a giant mess.