If Mars were to turn into a black hole, we would not be very happy since Earth (along with anything else remotely close) would get sucked into its enormous gravity, and thus be destroyed! However, we do not have to worry about that because planets, such as Mars, do not turn into black holes- only very large, massive stars end up as black holes after they die. Even our Sun isn't massive enough to turn into a black hole, and it won't die for billions of years, so we are completely safe.

Holly