I've tried the tech demo and it's a nice start.
About the movable block, I suggest giving very easy puzzle room for it, something like as soon as player enters, player is blocked by blocks. The one at north of player is movable while the rest is not. If player tried to interact with movable one, it will move north. If player do it again, it will move toward goal which is just 2 steps north of it. Doing this also allows player to get out from surrounding blocks and also the room.
Do the same with movable ice block. The purpose of this is to teach player about movable block and ice block without pressing them. The puzzle room with ice block could be moved elsewhere, near boss room if you want to :).
About the boss, the idea is good, I suggest giving animation when boss teleports such as blinking, phasing in or spawning smoke.
In general, first dungeon should be easy and gives introduction to mechanics your game has.