David Brin uses this in one of his books, playing a bit with some of the same kinds of ideas that gave us the Dark Forest theory as an explanation for the Fermi Paradox: intelligent species are out there, but nobody wants to be spotted because they’ll be killed preemptively.

This would take some intense pre-planning. So since you obviously can't capture a black hole to launch into a trebuchet, you'd have to do it to the original star before it collapses. Then time the launch so there's enough time for the collapse to occur during travel. You'd want to back up that launch time well before "just before collapsing" as the star's surface will have expanded to just be a tad bit unruly to handle. Better to grab it before that last bit of expansion just to keep things a bit easier to handle

> from exactly opposite sides and their force will cancel out

The gravitational force will only cancel out exactly in the center. Everywhere else, for example other planets, and when the Earth orbits, will easily detect the gravity.

> or consume the enemies host star.

It would take an infinite amount of time to do that, time dilation near black holes is intense.

Jeez man, peace in space!