It is true; there are two numbers, a "game" time and a "move" time. Blitz is 30/3, meaning each player has 30 minutes for all of their moves and 3 minutes for any given move. Go over either of those and you automatically lose the game. The number isn't displayed because it doesn't need to be. If you can't make a decision in 3 or 5 minutes, you should play with the "Slow" timer to the left of where you select a deck. Nothing wrong with that, I play with the slower timer often.
A bit of history, the game timer used to be 80/10 for everyone. When an opponent left the room without conceding for whatever reason, players often found 10 minutes too long to wait. MarcinS put the different timer options to accommodate different players, so those who want to get through 5 games in 2 hours can do so without brushing up against players who are learning or playing on a slow connection (which was a bad scenario for everyone involved).