OK, I think I’ve got it. Creating a calendar with no sites listed – an everything calendar – will block everything during times that are not selected. I think I was not waiting long enough for the router settings to take effect when I was first working on it.
An entirely blank everything calendar would block all sites all the time.
It’s difficult because it is the opposite of what I would expect. I think of the calendar as an active blocking strategy. My impulse would be to create a social networking calendar, and then block off the times I did not want them on social networks, not the other way around.