Where Are Hoohoo Blocks in Mario & Luigi: Superstar Saga?
They are not in this game. You are conflating two different series:
Hoohoo Blocks appear in Super Mario 3D World (Wii U / Switch). They are cubic blocks with a ghost face; hitting them spawns a Super Hammer power-up for a limited time.
Hammer is a standard Bros. Item/Action Command in the Mario & Luigi RPG series (GBA, DS, 3DS).
Mario & Luigi: Superstar Saga (GBA, 2003) – You obtain the Hammer early in the story (Hoohoo Mountain) after the tutorial section with the Hammerhead Bros. It is a permanent ability, not a block-spawned item.
Getting the Hammer in Superstar Saga
Progress to Hoohoo Mountain.
Meet the Hammerhead Bros. in the cave.
Complete their brief tutorial (smash rocks).
Receive the Hammer for Mario and Super Hammer for Luigi (later upgraded to Ultra Hammer).
There are no "Hoohoo Blocks" to find. The mountain's name is a coincidence.
