docstr: Moves this host to a blacklist, and starts the cooldown period.
docstr_tok: ["Moves","this","host","to","a","blacklist",",","and","starts","the","cooldown","period","."]
code: def blacklist(self):
      """Moves this host to a blacklist, and starts the cooldown period."""
      self._blacklisted = True
      now = time.time()
      if self._in_cooldown_period(now):
          return
      self._set_cooldown_period(now)
      self.set_event()
  
imports: import io
  import logging
  import random
  import threading
  import time
  from collections import defaultdict
  from horovod.runner.common.util import safe_shell_exec
  from horovod.runner.elastic.worker import HostUpdateResult
label_desc: Use image node_id 7 for calling the HostState obj's underlying member method code with example usage: obj.blacklist() without return types