Motors are painted out of the factory, if you want an unpainted look, there are paints that are made to look like the color of aluminum. The paint will give the engine a uniform color, protect it to some degree, help to cover up imperfections in the metal/casting marks.
You can blast it off and see if you like the look of it, but there might be some oxidation or something that might look unsightly. If that is the look you are going for though more power to you then.