Lord English is always associated with black holes because he himself represents a ‘point of no return,’ or an event horizon.
The Horrorterrors are masters over death, but explicitly in the sense that they surpass death and have power over the permanence of death. They have the ability to raise the dead, to preserve the forms of ghosts long after their bodies have died, and to twist and destroy the linearity of time (death) around them.
By killing the Horrorterrors and destroying the afterlife and its inhabitants permanently, English asserts himself as the point of no return, and enforces a permanence in death, for lack of better words. In this act he also, in a sense, breaks the chain of reincarnation, which is a very gnostic concept in itself.
The irony in this, I suppose, is that within Homestuck, black holes aren’t points of no return; they’re just pockets that set up the balls ready for the next game. So it might be less that Lord English is ‘like a black hole’ in that he represents an event horizon, and more that Lord English is the event horizon waiting behind every pocket to consume its spoils for himself.
(Also ‘consumes the sun’ is just a really simple allegory for ‘consuming the son’ which refers to Lord English’s Cronusesque nature).