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Check out
rostopic pub -h
For an explanation of --once. It doesn't do what you think it does.
Then, you might want to read the wiki documentation on msg, since it seems like you don't understand how you can use them. It might also help you to review the publisher/subscriber tutorial.
As for your question: std_msgs/Empty is just that - a message without any associated data. If you want to select which LED to blink, you'll want another message type that includes at least one data field.