Thanks, the reason for the black background is exactly as you describe it. It allows for showing what is interactive, as well as allowing me to draw characters of all kinds without risking things blending into the overall scene. I'm using a minimal palette for this, so it would be easy for a character with blue (like a blue fish) to blend into the ocean scene.
The how is simply a love for minimal pixel art and seeing a similar style at some point. I often find others who have come up with great ideas like this and then just take the concept to the conclusion that it could reach if the time and effort was put in to design a full pack.