I asked because i saw errors like
"$ORACLE_HOME/_ not found." "ksh: $AGENT_HOME/bin/emctl: cannot execute"

So i thought either ENV variables are not set properly or trying to run command with wrong user.

Well its strange that you can run "emctl status agent" from user but for same user while trying "emctl start blackout" its giving you emctl cannot execute.

You can also try to schedule repetative blackout from EM console itself.
(Just a workaround i thought)