I think the only reason Blockland cannot be on steam is because of the activation key system. Registering games on steam usually gives you direct access to the game with no activation code or ID system. Maybe, maybe, it could send a private steam message with the activation key or something like that. Just taking a guess though.
No, Frozen Synapse (coincidentally another torque game) also has auth keys.
Steam just pops up and says, "oh hey I copied your key to the clippboard, this is your key: [Key], feel free to paste it into any boxes that ask for it!"
There are many non-Valve games that do this. It gives a little pop-up the first time you launch the game, and then in your library you can right click -> view game cd key
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