Minecraft wiki entry for Polished_Blackstone_Bricks, associated text: "Cracked Polished Blackstone Bricks" is the current longest name for a block (excluding Education Edition), tied with Polished Blackstone Pressure Plate (34 characters with spaces, 31 without)
It briefly lost its place between Java Edition 1.17 snapshots 20w45a and 21w05a, where it was surpassed by variants of the Copper Block:
Waxed Lightly Weathered Cut Copper Stairs (41 characters with spaces, 36 without in first place)
Waxed Lightly Weathered Cut Copper Slab (39 characters with spaces, 34 without in second place)
Waxed Semi-Weathered Cut Copper Stairs (38 characters with spaces, 34 without in third place)
Waxed Semi-Weathered Cut Copper Slab (36 characters with spaces, 32 without in fourth place)
The record holders in the Education Edition are "Hardened Light Blue Stained Glass Pane" and "Hardened Light Gray Stained Glass Pane", both at 33 characters excluding spaces (38 including spaces).
The texture of polished blackstone bricks features a very subtle skull pattern. This is intentional.[1]