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.