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. The ship arrived in a low atmospheric orbit around Earth in the late 1960s, as the Sol system was located in the heading they were pulling away to. (TOS: "Tomorrow is Yesterday")