This block is not a random box. It instead has its own spawn rate and has special behavior: Each time you jump and hit the box, it produces money and jumps back to the right side of the screen. Each hit grants more coins, while also speeding up the box. Eventually, once you miss it, and if you hit it a LOT, it will burst in an event depending on how many times you hit it, with better events happening with how many times you hit it.
The design is a Blueish box with an X and a number behind it. Each time you hit it, the number goes up by one. So a combox that has been hit 3 times will show X3 on it.
Note: The name could also be the Safe, with changes to the sprite to match.