Things I liked:
You came up with a very clever concept, and for the most part your level design supports it. Once I got used to the controls, I quickly started to see the complex consequences of your simple premise, and I really liked the puzzles you made out of it. You did a good job of choosing side elements (blocks and darts) that augment your main mechanic, and because of that you got a lot of milage out of comparatively simple building blocks.
Potential areas for improvement:
Have you ever played Myst? Even though it's a completely different game, for me it shares the biggest drawback to your game; it's really hard to tell where you're going without animation. Because you didn't have time to create sfx or major landmarks within the dungeons, it was often very hard to tell in which direction or how far I was going. Otherwise, there isn't much wrong here, aside from the small scope and lack of polish that's expected in the average game jam game.
I'm very surprised that this is, as far as I can tell, your first game jam game. All things considered, the only real flaws here will be easily countered in the future by experience and practice. Given the creativity of your first outing, I sincerely hope you will continue this in the future, as I think you have a good shot at winning in the next couple of years.