Black holes, despite the implication that they swallow everything, even light, can be some of the most luminous objects in a galaxy... if they're feeding.
This is because the surface area of a black hole is limited, and the typical amount of dust and gas that gets drawn into an active hole can't fall in fast enough to simply vanish without a fight. Essentially, a matter traffic jam many magnitudes worse than all the rush-hours on Earth combined forms (an accretion disc), and the friction from all the jostled molecules and atoms struggling to dump their gravitational potential energy and inch that much closer to the black hole heats up the entire disc to ridiculous temperatures.
If the infalling matter wasn't already a plasma, it is now! And being plasma, it's beholden to all the peculiarities of plasma physics. The particles begin to interact and flow--magnetic fields are generated.
This is important, because once you're in the disc, you can't get out easily. You can't fall in faster because you hit those ahead of you in line, and you can't bust out the way you came because even though you're at a ridiculously high temperature and thus should have the energy to make an escape, there are way too many particles behind you that will absorb your energy before you make it very far. And angular momentum keeps you further locked down to the plane of the disc. Why? Think about those nifty experiments where you hold a spinning bike wheel and try to twist it around: that resistance you feel? That's what a particle has to deal with, except the accretion disc is spinning a hell of a lot faster than what even a dead Founding Father of the United States of America can muster in his grave.
The magnetic field propels particles out of the disc and toward the poles. As it turns out, a lot of matter is stolen this way, and so the ratio of what escapes the disc compared to what falls in the hole is surprisingly high. But what happens to all this stolen matter?
Think about a spring in a can. Someone before you has already used their strength to cram the spring in, and then successfully put a lid on it. Opening the lid would thus release the spring, and perhaps be cause for a lawsuit against the person who left a loaded spring in a can just lying around.
The friction of the accretion disc has crammed a lot of energy into the matter spiraling in towards the black hole, but the environment has done a good job holding that matter in check...until it gets chucked off by the field and reaches the poles.
The poles are weak points in the field that are the equivalent of opening the canned spring once this superheated plasma hits it. There's finally enough energy and enough of an absence of resistance for the plasma to explosively spew out both poles of the black hole at near light speed, like a hapless tourist who accidentally drank the water in Mexico.
The energy release is enormous. The matter at the poles shines across the entire EM spectrum, from gamma all the way down to radio waves. These jets of matter will travel for light-years, the grand pillars of radiative destruction slowly dissipating into the interstellar medium, but still "visible" large-scale due to the retained heat of the plasma. Any poor bastard star systems who find themselves within a certain distance of the originating jet will, as they say, "have a bad time."
It is this process that causes black holes to shine so brightly, but! Astute readers will notice that it's not technically the black hole that's doing the shining. The black hole is still black (I guess one can't go back), so it's the combination of the heated material in the accretion disc and the plasma jets that's doing the shining, marking the hole like a big damn beacon shroud floating out in space.
Thus, this makes being around a black hole a Very Bad Thing. You don't even have to fall into one to be killed horrifically; the heat and radiation from the matter falling into the hole will murder you long before you get to the hole itself.
If you actually want to have a chance at falling into the hole (and happily suffer the spaghettification process, which may be a preferable end for those who work call centers), you'll have to approach a black hole that isn't currently feeding, but you'd only be alive to cross the event horizon if the black hole is massive enough, pushing the horizon out past the point where tidal forces would pluck you apart like so many atom-sized gravitational chickens at the matter feed bag that is you.
