Moving From Placeholder To Game Art. Why I Started With Blocks Instead Of Using Art.
I have now strayed away from this :

To this.

The transition is simple with some caveats. My game is setup with 3D and is now transitioned with 2D art. So I needed to translate the Rigidbody Components to Rigidbody2D and converted any scripts using OnTriggerEnter() functions to its 2D counter parts.
Reason Why I Started With Blocks
I started with blocks because I personally find myself getting lost with the glam than focusing on the core gameplay loop. I find that working with blocks allows me to hone in on what I needed the core gameplay to be and to measure if its actually fun to play. I can quickly test an idea, move it forward if I find it interesting, and then decide. Since this is a concept that has been done throughout the ages, it was a no brainer to push it forward.
In the past, I have worked on projects I learned that weren't fun after investing a lot of time into it. That project was bringing the Fire game from the anime Fooly Cooly (FLCL).
In short, Keep it simple, work layer by layer.