The other factor is Old!Calliope. Her existence also smells of deus ex machina, by virtue of her presence not entirely being explained. How did Calliope gain powers par Lord English, when, i'm sure I forgot, but she wasn't prototyped with a guardian, sprite, another player, an insidious calculating pair of glasses and a guaranteed immortality from everything? How is she able to create a black hole out of nowhere that is equally powerful to that of the green sun, when she isn't basically Lord English? Jade, being prototyped with a guardian, doesn't lose her powers and manages to resize the world back to normal, when the green sun is a constant in all universes?
Old!Calliope was a fully realized Muse of Space. Black Holes are just the
compression of matter into such a small space that reaching the escape velocity requires infinite or greater effort, (i.e. light speed or greater) thus creating a perpetual and irresistible collapse. Heck, Jade might very well have been able to do it, but even if she couldn't, I figure Muse as one of the two superclasses probably could. Calliope wasn't as powerful as LE, but she was working well within her sphere of influence.
Furthermore, I suspect a black hole wouldn't need to 'overpower' the Green Sun. The Green Sun was
immense. Collapsing into black holes is what immense stars eventually do naturally anyways. Effectively, she was using gravitational judo to use the Green Sun's mass against it.
And as pointed out, Jade's resizing powers are class-related, not from being part First Guardian. Presumably the Green Sun wasn't amplifying her abilities, though I can certainly see why you'd think so - nobody else had aspect powers suited for working on that kind of scale. Even John was stuck within the limits of the planetary atmosphere. (Though he did manage to core Skaia with it, so he's clearly got some oomph.)