A black hole's gravity is so strong it can swallow everything, even light. Our galaxy is full of black holes, there are about 10 million black holes in the Milky Way, or so scientists estimate. The black hole wouldn't even have to come very close to Earth to cause catastrophe, even by goin through the solar system it would ruin the planet's orbits. Black holes are said to be made of pure gravitational energy and are actual holes yet act like stars, if our Sun would be replaced by a Black Hole we would still orbit it, yet recieve no radiation or heat. Since Black holes orbit just like other stars,  it means that they probably aren't headed our way. However if a star was moving towards Earth we would see it, but a black hole would be nearly invisible. Scientists would probably have a few decades warning before a black hole hit Earth. If a Black Hole went through our universe and went near Earth it would make Earth expierience extreme climate swings, or it may lose its orbit around the Sun and end up floating away into dark space, into nothing but death.