A black hole is a concentrated singularity of gravity caused by a dead star collapsing in on itself, which absorbs everything within its reach, including light. Black holes cannot be seen, but the accretion disk, which is the flow of matter being pulled by the black hole, is visible. As one gets closer to a black hole, time appears to slow down, but since time is relative, it is only slow compared to those who are viewing it at a "normal" rate. (SG1: "Singularity")
While dialing P3W-451, the SGC was caught by a black hole consuming the dialed planet. If the connection had remained active, Earth would be swallowed by the black hole. As the black hole got closer, time slowed down, causing the SGC to lose contact with the rest of the outside world. The connection was broken when an explosive device was detonated close to the Stargate. This caused the wormhole to jump to another nearby gate and it was then able to be shut down. SG-1, using Ancient technology Ba'al had somehow gotten his hands on, were later able to return to the planet in a ship and rescue SG-10 who were going to fall in. (SG1: "A Matter of Time", "A Matter of Honor", "The Cost of Honor")
Due to the interference of the Goa'uld Hecate, SG-1 were lost when she was able to program the Stargate SG-1 were about to use so that the wormhole to Earth was redirected around the elliptical orbit of a black hole, with the result that SG-1 essentially remained 'in transit' for almost a century. Upon emerging in the future, Colonel Jack O'Neill and Major Samantha Carter were able to use a time-travelling gate-ship they found in Atlantis to go back in time and prevent Hecate diverting their past selves, but this only created a new timeline rather than erasing the one they were currently in (Stargate SG-1: Apocalypse - Insurrection).
In order to cause Vorash's sun to go supernova and destroy Apophis' fleet, SG-1 connected Vorash's Stargate to the black hole near P3W-451 and sent it into the star, sucking some of the sun's matter into the black hole through the wormhole before the Stargate was destroyed and causing it to go supernova, obliterating the fleet but throwing SG-1 and Apophis into another galaxy. (SG1: "Exodus")