"if there is a signal fluctuating between 4.5V and 5.5V before the cap, it'll be -0.5V and 0.5V behind the cap" if there's a resistance to ground after the capacitor. I've accidentally planned an audio amp with input decoupling capacitors, didn't add ground resistors before the capacitors, so if you plug it into an audio output with DC-blocking capacitors but not resistors to ground, the voltage in the middle could be anything.