How can a blackhole harm us?
Answers
To be precise, one black hole Sgr A* has its effect on us. Sagittarius A* is a bright and very compact astronomical radio source at the center of the Milky Way, near the border of the constellations Sagittarius and Scorpius. Sagittarius A* is believed to be the location of a super-massive black hole, like those that are now generally accepted to be at the centers of most spiral and elliptical galaxies. Our Solar system is believed to be rotating it.
There are several ways that black hole can affect us.
1- It distorts light coming from its background due to immense gravitational pull causing gravitational lensing effect thus there will be erroneous astronomical data.
2-Black hole singularity is a disaster for physics (for now) as it requires a reconciliation between general relativity and quantum theories.
3-Gamma Ray Burst: Gamma Ray bursts are not good for Earth! It will fry Earth if it is created at close distance. Most observed GRBs are believed to consist of a narrow beam of intense radiation released during high-mass star collapses to form a black hole. Luckily we are out of their range.
4-From near, it can pose serious damage to us. It will devour the solar system and the sun and we will have a last good day on Earth. Once the Black Hole came within about 1 AU of the Earth, the gravitational tidal forces would rip the Earth itself apart.