...
ssh root@backupserver
ssh backupserver
ssh bl@192.168.28.2
Gotcha: the first time such a command is used for a particular host, ssh prompts interactively for confirmation to proceed. When confirmation is given, it adds the host to ~/.ssh/known_hosts. Thereafter there is no such interactive prompt. If the passwordless login is to be used from a script, it needs to be done manually the first time or the script will hang.
In case you prefer a password protected key or you want to "hop" from system to system, SSH Agent Forwarding is helpful.