A black tank is a large chamber pot. It isn't a septic tank where the waste is turned into compost. Instead, what happens is that TP and poop should be turned into liquid. With this in mind, it doesn't matter what bugs are in the tank. Because of this, it doesn't hurt to bleach it out to get rid of odors.
As for turning waste into liquid, the magical chemical I use is Dawn hand dish soap. It dissolves grease off of dishes, and when dumped in the black tank, it does quite well in doing the same exact thing to stuff in the black tank. I've been doing this for a few years now, and so far, no clogs or plumbing issues, so I'm either lucky or doing something right.
Plus, there isn't anywhere else to put the water from sanitizing the tanks. If I dumped the bleach water onto the ground, I'd leave a soggy mess of dead grass for the next person. So, I might as well have it kill odors (especially in the grey tank that can get quite stinky) and call it done.
