Yes, I know this is Adventure Time. However, there is something to keep in mind with what Finn is doing. It is true, that each dimensional object casts a shadow one dimension less than the actual object. As silly as it is to learn this from a Cartoon, it's a great way to think about dimensions as a whole.
Is this true? Yes, it is. This particular clip plays off the idea that the fourth dimension is Space-Time itself. Thereby making it cast a 3-Dimensional shadow, which obviously would mean all around it. That's why they theorize it as a black hole. Because it has to block certain aspects of that dimension, or at least dull them, in order to cast a shadow.
Now, The real question to ask then is: Are black holes a dimensional object we cannot perceive due to the very nature of it? Or perhaps, are Black Holes themselves a Shadow of a 5-Dimensional object? We do know that Black Holes tend to stump us on how they can bend physics from time to time. Relativity is great, but it doesn't explain everything about them.
So my thought is, if they are a shadow from a Five-Dimensional Object, what is the Fifth Dimension? Could it be reality itself?