How to make a black hole in FS2 (you need fs2_open for the $NoGlare part to work):
1) Make a black (RGB: 0,0,0) circle on a green (RGB: 0,255,0) background with no antialiasing in your paint program, and save it as blackhole.pcx (make sure it is 8-bit colour) in your effects folder.
2) Make another image, this one just all black. Save as reduce_ambient.pcx in the effects folder.
3) Make an entry in the first part of stars.tbl like this:$BitmapX: blackhole
4) Make an entry in the second part of the stars.tbl like this$Sun: reduce_ambient
$Sunglow: reduce_ambient
$SunRGBI: -1.0 -1.0 -1.0 -1.0
$NoGlare:
5) In Fred, place blackhole.pcx as a background image (scaled to whatever size you want) and reduce_ambient as a sun. Put them at the same co-ordinates.
Tada!
(Note: You won't be able to see either of blackhole.pcx or reduce_ambient in Fred since they are both black. In FS2, you'll get a big blank spot in the sky and lots of darkness.)