This is a fun thought experiment, but it's hard to accept the premise, since it seems less important that we agree to coordinate around one single time value, vs having the semantics of time be broadly equivalent, e.g., 5am means nearly the same thing to someone in Tokyo as it does to someone in Omaha.

It would impose significantly more cognitive burden on me to try to imagine the perspective of any random person speaking about time than it does to have to correct for multiple time zones, which I have tools to help me do whenever needed; and I expect I have a more challenging multi-time-zone coordination problem than most.

To make this even more explicit, imagine the same issues in play, except for seasons. Right now, in North America, we're moving into winter; but in Uruguay, you're moving into summer. We could just agree to take Uruguay as a reference, but now "summer" means vastly different things to you and I, even if the term is literally describing the same universal time. The seasonal affordances of the world predominate in human affairs over the absolute time in which those affordances unfold. Blocktime loses all those distinctions.