after all this the only way to detect such a ship would be to have a very wide feild camera that would look for stars disapearing in the back ground as the ship moved infront of them. this would work well at close range but at long range would become very difficult as the "veiwing" angle needed to see the disapering stars got very small you would need extreamly fine tuned equipment.
And you can also assume that they would be able to create an image on their hulls to make it seem as if the star is really there, my point was that if you have a fleet of them, a sensor is bound to notice that something is wrong, at which point everyone would just send as loud of a pulse out as possible, the same as active pinging for sonar. It would then be simple to determine where the ships are, especially if there was more than one ship sending out the ping.
Also, we have found black holes using the same method as you described. While it is true that black holes are much bigger than a ship, the distances involved are much smaller and the viewing equipment and, more importantly, the computers interpreting the data are far more advanced, making it nearly impossible to do such a thing.
All radiative energy can be manipulated via gravity, black holes for instance do not just suck in visible light, they manipulate the entire electromagnetic spectrum.
as to this, we ALREADY have ways to detect gravitational warps and we cannot yet warp gravity. To say you can just magically suck everything and hide would probably be the best way to get yourself found, even if you somehow managed to keep that immense gravitational pull limited to a very small radius outside the ship, they would probably notice that all of a sudden there is *nothing* coming from a certain direction. Already, a way has been found to detect new-age American subs, they are *quieter* than the background noise, so you just look for the 'black hole.'
