Cubemap is not map for cubes. Cubemap is for skybox.
Your cube has no uv map for the texture to be applied. You would need to use a 3d model software for that.
The uv map would define that vertex 0 is pinned to a 2D coordinates on the texture like (0.33, 0) as UV map is normalized. then vertex 1 would get (0.66,0) and so on.
At the moment there is no uv map so each face gets the whole texture with black to fill in the blank (as you probably have no transparency).