'#' is now added automatically to the beginning of the texture names in B3D models, and the remapping is expected via the blockstate JSON, since it's more reasonable than adding it to the file name in the modelling program or matching the resource location with the filename.

src/main/java/net/minecraftforge/client/model/b3d/B3DLoader.java

@@ -354,7 +354,9 @@ public class B3DLoader implements ICustomModelLoader
             for(Texture t : textures)
             {
                 String path = t.getPath();
-                builder.put(path, new ResourceLocation(getLocation(path)));
+                String location = getLocation(path);
+                if(!location.startsWith("#")) location = "#" + location;
+                builder.put(path, new ResourceLocation(location));
             }
             return builder.build();
         }