The result of it is:
$AGENT_HOME/bin/emctl start blackout PRUEBA_AGENTES "srvoem1a.garba.com.ar:3872":agent -d 30
With emcli command I don´t find the agent type, I have found the group type, so I have created a group with two agents, then I created a blackout on it, but the agents continue in active state.
Blackout start Error : the agent target "srvoem1a.garba.com.ar:3872" does not exist
I have put a node level blackout on OMS´s server, but the alerts of "Agent is unable to communicate with the OMS" continue. So I think that an option would be put a blackout on all target hosts. Because I need down oms to take a cold backup.
So, I will investigated how to put a blackout on all targets automatically, and then I will post it here.