Black holes can be used to travel into the future only. So far as we know, our universe prohibits traveling into the past. According to Einstein's theory of general relativity, and to experimental evidence here on earth assembled by Harvard physicists Pound and Rebka, in the presence of a gravitational field, an external observer would see a clock in a strong gravitational field tick more slowly. This is analogous to the famous time dilation effect in special relativity, except that in the 'gravitational redshift' no motion between the observer outside the gravitational field and the clock located within the field, is required. What this means is that if you were traveling into a strong gravitational field and sending out pulses of light every second, an observer watching these signals from a great distance would see the interval between the pulses increase from seconds to minutes and then hours.
Black holes are fantastic sources of very strong gravitational fields.
What a distant observer would see as your clock got closer to the so-called Event Horizon of the black hole is that the pulse interval would increase without limit from one second to one month and longer. The frequency of the light pulses would also get longer as the light lost more and more energy struggling to get out from the vicinity of the black hole. So, if it took your friend 1000 hours to travel from where you are to the black hole, the last photon he sent you just before entering the black hole, would arrive at your location 1000 hours from now, but when you looked at the interval between the last two pulses he sent, you would see that they are not the one second interval you started out with, but say 1 or 2 minutes or more. But here's the rub. In other words, one second to your friend falling into a black hole is several minutes to you and, in essence, your friend is aging more slowly than you and is traveling into the future faster than you are.