Alright, the GalaxySenarioDef file has several sections, two of them which are required to be modded for completely new planets. Here's the beginning of what the first section (might) looks like (minus formatting of course).
orbitBodyType "Terran" ...
The planetTypeCount is the line that appears first in the planetType section. It simply tells the game how many different planetType's their are so it can read the file correctly. Have the count equal to the number of planetType and this won't cause you any problems. The DesignName is simply what the planetType name is and from what I can tell is only used in the GalaxyForge (or editting .galaxy files by hand). I'm honestly not sure exactly where the string is used, probably galaxy forge as well, but honestly I don't think you have to use it. OrbitBodyTypeCount is important, like the planetTypeCount its used to get the game to read the file correctly. It is simply a count of what all the different orbitBodyTypes for each planetType. Again, make sure the number of orbitBodyType lines equals the count and it shouldn't cause any problems. The orbitBodyType is the name of the planet (or star) that you made and is defined in the next section.
For what I think you're trying to do, you'll probably want to create a new planetType (usually done right below the last one) that only contains an orbitBodyType for your black hole so you can use it in galaxy forge. You'll also want to add the black hole to the RandomUncolonizable planetType so that it can appear in the current maps. Remember to update the counts when you do this!
The next section is the orbitBodyType section, which tells the game where to find the files for the different types of entites we used in the last section. It will look something like this.
The orbitBodyTypeCount works like the planetType count except its for the orbitBodyType. The typeName is the name that is used for the other parts of the GalaxySenarioDef file (so if you call your new planet "Black Hole" make sure its always spelled like this for the rest of this file). The entityDefName is the name of the .entity file that contains your new planet (you obviously don't need to include the .entity at the end). The defaultTemplateName tells if the planet type will have a default template (usually used to spawn militia or pirates) that will load at the start of the game. If you don't want to use one, keep it like it is above.
All you should need to do is add a new orbitBodyType, fill in your planet's info, and then update the counts. Hopefully that should get the game to start including the planet type in its maps (provided everything else works).