Paths are wrong and have a double slash.
We define the $blocks_dir var like this:

$blocks_dir = dirname( __FILE__ ) . '/../build/block-library/blocks/';

then when we build the path for the block JSON file we do it like this:

$block_json_file = $blocks_dir . '/' . $folder_name . '/block.json';

Doing an error_log( $block_json_file ) shows that paths are formed like this:

wp-content/plugins/gutenberg/lib/../build/block-library/blocks//column/block.json

$blocks_dir already has a trailing slash, so the extra slash between $blocks_dir and $folder_name should be removed.