A black star is a stellar object which has high gravitational attraction. Black stars can remain undetected until encountered by entering their gravity field.
In 2267, the USS Enterprise, en route to Starbase 9 for resupply, became caught in the gravity well of a previously-uncharted black star. The Enterprise was able to break away by using all warp power in reverse, but the resulting whiplash effect propelled the ship into a time warp, transporting it to the late 1960s. The crew of the Enterprise was eventually able to return to their own time by recreating the slingshot effect which transported them through time in the first place, by using the gravity of the Earth's sun to recreate the time warp. (TOS: "Tomorrow is Yesterday")