"""Extracts the content of a block enclosed by curly braces from a given text starting at a specified index.

This function searches for the first occurrence of an opening curly brace '{' starting from the given index and extracts the entire block, including nested blocks, until the corresponding closing brace '}' is found. The function handles nested blocks by maintaining a count of open braces.

Args:
    text (str): The text from which to extract the block content.
    start_idx (int): The index in the text from which to start searching for the block.

Returns:
    str: The extracted block content, including the enclosing curly braces. If no valid block is found, an empty string is returned.

Note:
    - The function returns an empty string if no opening brace is found or if the block is not properly closed.
    - The function is case-sensitive and only recognizes '{' and '}' as block delimiters.
"""