-Scale it up 0.1 (the more your scale the thicker the blackness).
Actually, to create a consistent outline, you need to scale each vertex of your duplicated mesh outward by a set amount rather than scaling the mesh itself. But even using this method, you'll get a very "dirty" looking outline that will appear thicker the closer the model gets to the camera.
I don't think that anyone has come up with a pretty method of outlining meshes in Blitz3d yet.