Here is a bit more information on the typical astronomer’s thought process as well as properties regarding typical observation methods.  This also explains why it is tricky spotting them:

As the stars, gas, and dust in the center of a galaxy get closer and closer to a supermassive black hole, they get packed tighter and tighter into a smaller and smaller space, heating up until, at a critical distance, everything is ripped apart, reduced to atomic particles. When we spot supermassive black holes, it’s this heat radiating away from the orbiting debris—known as an accretion disk—that we actually see, not the black hole itself. Some supermassive black holes “eat” more than others and, in the process, give off significantly more light than their less active brethren. These “active galactic nuclei,” or AGN for short, are some of the most powerful, most energetic forces in the Universe. Not only do they give off heat, they also often eject material in the form of collimated (beamed) jets, perpendicular to the plane of the disk, which blast their way out of the galaxy’s core—dwarfing in size not just the accretion disk, but also the galaxy itself. What’s more, some AGN have a dusty torus, the geometric equivalent of a donut, in the same plane as their accretion disk, but much, much bigger and thicker. So thick, in fact, that if you looked at them from the side, you wouldn’t see the disk at all, much less the black hole in the center.

The light we see doesn’t always paint the same picture.

The author of this article also makes an astute commentary in that we only have one vantage point to compare and that is from earth:

There could be other factors at play than just our point of view, like physical processes in and around black holes we don’t fully understand or measurements we haven’t thought to take. As we build better telescopes and amass new data, we can only hope that we’ll see these active galactic nuclei for what they really are. 

thanks to nautil.us for the great info

thanks to NASA for the pic