"And when I reach out into event horizons, will there be light and sound, or will it be just me."
An event horizon in general relativity is a theoretical boundary or a "point of no return", where the gravitational pull is so great it's impossible to escape, making all events that occur beyond it ineffectual to an outside observer.
An excerpt from Wikipedia says that:
"An event horizon is most commonly associated with black holes. Light emitted from beyond the event horizon can never reach the outside observer. Likewise, any object approaching the horizon from the observer's side appears to slow down and never quite pass through the horizon, with its image becoming more and more redshifted as time elapses."
We can possibly take this to be "the edge of the explosion", an event horizon. Linking this to various points in In The Blind, we have a mention of redshift, "redshift bathes the hours when it's weakest", and a mention of a vantage point from the edge of the explosion.
We can possibly link this to Prologue III (Echodeath), where "the final sample spun and cancelled all her work" can link the the spinning cycles in In The Blind, which will cause there to be nothing beyond the event horizon. We also have the link with "And the sun set for one final time over the Echoplane", where there would be no light or sound beyond the event horizon if the Echoplane was destroyed.
There may also be a link with "Do you, do you want to live inside the hive? Or do, do you want to die on the outside?" With the hive being the place (Echoplane) within the event horizon, and those on the outside will not see inside due to observers not experiencing effects. As the event horizon is "the edge of the explosion", those on the outside will probably die in the explosion.
Extension: World Lines (The 'Blackline and Underline Are Non-Linear Timelines Because Physics' Theory)
"A single electron moving in all directions and all times"
There is a theory that there is only one electron in the universe, and it moves through spacetime to the point where it is present at every location in every time, used as an explanation as to why all electrons have the same charge and mass. According to Wikipedia:
"The idea is based on the world lines traced out across spacetime by every electron. Rather than have a myriad of such lines, Wheeler suggested that they could all be parts of one single line like a huge tangled knot, traced out by the one electron. Any given moment in time is represented by a slice across spacetime, and would meet the knotted line a great many times. Each such meeting point represents a real electron at that moment."
A world line is the unique path of an object through 4D space (spacetime), hence the electron leaves an imaginary line throughout spacetime. However, as world lines represent a particle moving through time and not space (to include space dimensions would form worldsheets and world volumes), physics rules apply whereby a larger object can be equated to a point or a particle. That is to say, that humans can be considered a particle, and their timeline of events can be shown as a world line. The implications of this are, when related to the Lines theory, that Blackline, Underline etc could all be different timelines of Cassandra or Sparkles*. They may not fit onto a specific linear timeline, because they're not supposed to be on the same timeline.
Another interesting followup from this is the existence of a type of world line called a closed timelike curve; a world line that always returns to it's starting time as it moves through spacetime, effectively closing the curve in time, but not necessarily in space. If a CTC passes through an event horizon, a phenomena known as chronological censorship occurs, whereby an outside observer would not be able to see a causal violation caused by a CTC.
"Ordinarily, causality demands that each event in spacetime is preceded by its cause in every rest frame. This principle is critical in determinism, which in the language of general relativity states complete knowledge of the universe on a spacelike Cauchy surface can be used to calculate the complete state of the rest of spacetime. However, in a CTC, causality breaks down, because an event can be "simultaneous" with its cause—in some sense an event may be able to cause itself."
A closed timelike curve breaks causality, but event horizons mask the violation. Therefore, if the 'single electron moving in all directions and all times', causing a CTC world line, passed through an event horizon, it would appear to an observer as if no violation has occured, yet the very existence of that electron is a violation of causality.
I like physics. <3