Go to the ticket booths, show your AP and ask for a blockout day ticket. I think they're $30. Not sure though. If it's a blockout day, I think you'll have to pay for parking also, even if you have parking on your AP.
Yes you can parkhop but you'll have to show the blockout ticket everytime you enter a park.
If you upgrade to a higher level AP, then you can also enter the parks on that day even if it is a blockout day. You'll have to show the receipt everytime.