Are the black chained squares the key gate in Level 21?

No. They remain lock-marked physical blocks and serve as movable temporary platforms for the climb.

How do I reach the raised door?

Jump onto the three blocks in sequence and reposition them into a rising cluster below the upper-right door, then make the final climb.

Mr Potato level guide

hard

Use the three chained black blocks as movable footholds, build a rising route, and climb to the elevated upper-right door.

Layout: Mr Potato starts on a long white floor in a grey grid room. Three black square blocks with chain-and-lock symbols sit at separate floor positions, while the arched door is raised at the upper right.
Goal: Move the three black blocks into a climbable staircase and enter the elevated door in the upper-right corner.
Opening: Move toward the nearest chained block and jump onto it so the first block becomes the starting foothold for the airborne route.
Danger Zone: Missing a block drops the potato back to the floor and can break the temporary arrangement; the upper-right block cluster also requires careful alignment before the final door jump.
Mechanics: The chained-looking blocks are movable physical platforms. Repeated jumps onto them change their positions and let the player assemble a rising route; no key or enemy is involved.

Treat the three black blocks as a route built one foothold at a time rather than trying to jump directly to the raised door.
If a jump falls short, rebuild the lower block position first and use it to regain the staircase.
Think in chain clears. The best move is the one that sets up the next two moves, not just the quickest current match.

Move from the starting floor toward the nearest black chained block and jump onto it.
Use the first block as a foothold while shifting the next block upward and to the right.
Jump between the three black blocks, keeping the potato on the lower block whenever the upper block is still out of reach.
Continue raising and spreading the blocks until they form a staircase-like cluster below the upper-right door.
Make the final jumps across the upper blocks, align with the raised door, and enter it to finish the level.

Trying to jump from the floor directly to the upper-right door skips the block route and leaves the potato below the exit.
Jumping past the last block without lining up first can drop the potato from the upper cluster and force the climb to be rebuilt.

Are the black chained squares the key gate in Level 21?
No. They remain lock-marked physical blocks and serve as movable temporary platforms for the climb.
How do I reach the raised door?
Jump onto the three blocks in sequence and reposition them into a rising cluster below the upper-right door, then make the final climb.