In resource packs, smooth basalt is categorised as a custom block, so it can't be customised using the blockshape element in blocks.json.

Example pack: https://drive.google.com/drive/folders/1u4liKlnvGxPvkHtE1Q-HQNsiufnmvmlI?usp=sharing
In this pack, both basalt and smooth basalt have blockshape set to fence, but only basalt gets the block shape and there is the content log error "[Blocks][error]-'minecraft:smooth_basalt' | Cannot set 'blockshape' on custom blocks."

contents of blocks.json:

{
"basalt":

{ "blockshape": "fence" }

,
"smooth_basalt":

{ "blockshape": "fence" }

}