I am aware of no real way to unblock them in the (reasonable) ways you suggest.
Beware: entering serious "work-around" territory here...
So you can block an appointment two ways. Go through admin options which tends to be better for full days/weeks off.
Or, use the "Block" button on each days schedule for shorter blocks like a meeting. It sounds like that is what you want to "unblock".
Once the "block" is on the schedule, I don't think there is any way to erase it, but you can schedule over it; a new patient can be put in the time slot. Leaving the slot blank to indicate it is open would be ideal, but since you can't do that, create a patient with the name you want to put there. You could just use Mr. Open, or some such. We want the appointment slot to be as visible as possible, so we have a patient named "Mr. .." for this occasional purpose. That's right: first name is "." and last name is ".". Schedule that patient to unblock the time slot and when you glance at the schedule, it is pretty clear it is open. Just "change appointment" when you want to put someone real there.
