Teletext used the vertical blanking interval too. It consisted of numbered pages of text, each number input on the numpad on the remote control as a form of hypertext. Page after page was transmitted after one-another, repeatedly. It was sometimes used for subtitles, with some pages with a transparent background. Better receivers cached pages so you wouldn't have to wait for its next transmission when going to another page...

Digital television formats adopted the framing from analogue formats and sends the same data in digital form within the vertical blanking interval. Many channels have stopped offering teletext. One network here in Sweden still uses it to deliver news, and I often prefer that format because the articles are concise and distraction-free.

BTW. I was once asked to hack together a system for using data in the vblank period to control relays at a remote site.