It can be done for outbound calls. And when the Smart Block is on during the times you specify, the user does still have access to 9-1-1 calls. That was my primary question when I signed up for the service.
From the support location here on AT&T: Select the days and times that you want to block messaging, browsing and outbound calling.
We're sorry, but this feature cannot currently be used to block incoming calls.
Reminder: All incoming calls are allowed at all times except from Blocked Numbers.