Prior to the mid-1990's, blanking the screen simply meant painting the screen black in software. Later video cards would allow you to turn off the video signal and monitors would blank if they lost the signal. However, this would only work if both ends supported power saving modes. Otherwise the electronics (including the picture tube) for the monitor would operate normally. The only other option was to turn off the monitor manually or, on machines with an integrated monitor, to turn the brightness down.

If the monitor is going to be on anyway, why wouldn't people want to display something interesting on it?