CG Artist Vito LaManna creates some pretty great BMD Fusion tutorials. If you haven’t seen any of them, you should check them out. His latest shows how to use refraction inside of Fusion, to create a nice looking 3D black hole effect.
The effect is created with the shape3D and Displace nodes, and some refraction. The thing is, refraction in Fusion (much like reflection), is environment based. This means that they will only show up if an environment map is used. Using the sphere map tool in Fusion helps to marry the refractions and the 3D displacements together.
In all, the black hole effects is a neat effect (that could be reproduced in any other compositor to some degree), and there are a lot of tips to glean from this one project. Great stuff!