Currently, the sqlite3 module does not correctly handle cases where a Blob object is accessed after its parent database connection has been closed.

Expected Behavior
To match CPython's behavior, attempting to read (or perform other operations) on a blob after calling connection.close() should raise a sqlite3.ProgrammingError with the message "Cannot operate on a closed database".

Context
This issue prevents the standard library test test_blob_closed_db_read from passing. We need to add a check to ensure the connection is still open before processing blob operations.