Actually, the Enterprise, or any other warp 9+ capable starship (say, a Klingon Bird of Prey) can travel through time without any black hole. It only takes a regular ol' star, like, say, our own Sol, and the correct trajectory and speed.
Source: Star Trek: TOS: Tomorrow is Yesterday
Source: Star Trek: TOS: Assignment Earth
Source: Star Trek IV: The Voyage Home
That is, using a Starship. There are other methods. The Guardian of Forever, for example, or the Atavachron of Sarpeidon, temporal disruptions, cold starting warp engines to exit a gravity well, Q, TARDIS, H. G. Wells' Time Machine...
But at some point, apparently, Starfleet stopped all that crap (and buried it, one supposes), so it wasn't used again in any other cases beyond the repopulation of Earth's humpbacked whales. One shudders to think what might happen if, oh, say, a temporally unethical bunch of people were to start using it...
Oh, right. Temporal Cold War. Yeah. :p