Biome blocks are blocks that form the ground of the biome. For example, Garden biome always has Cobblestone, Dirt and Mud.
Biomes can't have other blocks (only those from their "biome blocks" list). However, a biome can have microdungeons, and those microdungeons can have other blocks. For example, Garden biome doesn't have Plant Matter, but its microdungeons often do.
Most biomes have only 3-4 types of blocks. Underground Tidewater biome has the highest number of different blocks (8), while Gleaming Asteroids and Silt Wonderland biomes have only 1 type of block.
List of biome blocks