And teens today (or soon) would not visually know what "television tuned to a dead channel" meant...
It's almost more profound in some ways - on a lot of modern TVs, a dead channel is displayed as being a solid field of bright blue. So, it changes the way Sprawl's sky appears to each reader, but it makes the change take place in Case's head. If it's bright blue, and a person recognizes that first as related to a TV, what does that say about the way they perceive the world? Especially with how bad Case wants digital access.
Definitely the greatest line I've ever read. Such extensive imagery and metaphor (well beyond just the sky), tucked into 15 words.