During the events of Marathon 2: Durandal, Blake was a Commander of the human forces on Lh'owon. The men under Blake's command rescued the player from imprisonment by Tycho and the Pfhor. Blake later directed him in locating and activating an ancient S'pht AI, having taken over when it appeared that Durandal had been subverted or destroyed.
At the end of Marathon 2, Blake and his men escaped Lh'owon's destruction and returned to Earth in their captured Pfhor refueling ship, the "Hfarl." They were the only human survivors of the original Tau Ceti colony.
Blake's actions were seen from a slightly-different angle in Marathon Infinity.
Trust me, I'm not a Pfhor-lover, but with Durandal no longer pulling the strings a great weight has been lifted from our shoulders. We're planning to steal a Pfhor ship of our own and return to Earth.
You're invited to come with us when we do.
- Commander Robert Blake, Marathon 2, God Will Sort The Dead..., Terminal 1: 2nd message
Robert Blake left something for you a few minutes ago. He and the other humans captured a Pfhor refueling ship and found or forced someone to pilot it for them. They folded out of the system immediately after I received the message, without so much as a "Thanks for saving us from becoming plasma on Tau Ceti, Durandal."
Anyway, the message reads: "The dead walk again; we cannot wait. -Blake." I think he means me, but I let him go anyway.
- Durandal, Marathon 2, Fatum Iustum Stultorum, 2nd Terminal
The player actually does meet Robert Blake in the level Where the Twist Flops (Level), he is the only blue bob (science bob) on the level. He looks no different then any other science bob and has an eye implant, contrary to the chapter screen and terminal pictures. He can also be killed, but it is of course meant he does not.