It wasn't really meant for anyone in particular.
You can sell it to kids because it doesn't have anything inappropriate, because it's literally just a bunch of squares and rectangles that you can use to make stuff
Same goes for adults, you can sell it to them because it doesn't have any child-oriented content
It's a lot more noticeable that Blockland in it's original form was intended for a younger audience, as opposed to adults

Where as Blockland now doesn't really fit either demographic entirely (stuffty image quality I know)

Sure it's blocky and stuff, but that shouldn't be the only dictator if a game is for kids or not. Just look at something like Doom. I don't think you should give little Johnny a copy of Doom for Christmas