input: from typing import List
  
  def create_blockchain(blockchain_transactions: List[dict]) -> List[Block]:
      """Create a simple blockchain system where each block contains an index, timestamp, data, and a reference to the hash of the previous block.
      >>> create_blockchain([{"transaction": "data1"}, {"transaction": "data2"}])
      """
output:     class Block:
          def __init__(self, index, timestamp, data, previous_hash):
              self.index = index
              self.timestamp = timestamp
              self.data = data
              self.previous_hash = previous_hash
              self.hash = self.calculate_hash()
  
          def calculate_hash(self):
              block_string = f"{self.index}{self.timestamp}{self.data}{self.previous_hash}"
              return hashlib.sha256(block_string.encode()).hexdigest()
  
      import hashlib
      import time
  
      blockchain = []
      previous_hash = "0"
  
      for index, transaction in enumerate(blockchain_transactions):
          timestamp = time.time()
          block = Block(index, timestamp, transaction, previous_hash)
          previous_hash = block.hash
          blockchain.append(block)
  
      return blockchain