There is no such thing as stealth in space. Anything coming from the moon to the Earth would either need a huge amount of thrust to go straight down (which would be easy to detect), or would cross into daylight and reflect a lot of heat even if it were black. And you can bet that anyone who had a launch capability on the moon would be under close scrutiny.
They'd also better have a first-rate missile defence shield, because in the two days it would take their projectile to reach Earth, their target would launch a lot of ballistic missiles (which take a maximum of about 40 minutes to arrive, anywhere in the world) and render their cities smouldering ruins.
And if they're relying on a large mass, not a warhead, then they won't have much delta-V for dodging, so they'll be easy to intercept. If they've painted it black, then it will be very vulnerable to heating by laser weapons and whatever colour it is it will be easy to fragment. The kind of mass that you'd need to survive reentry would either need to be huge or covered in (very fragile and vulnerable to attack) ceramic tiles to dissipate the heat. If you fragment it, the atmospheric friction will cause the entire thing to evaporate.
If attacking from space were easy, then Earth would have had a lot more extinction events in its past. The atmosphere alone does a pretty good job as a meteor shield.