The last couple rows in the back are blocked because DL is holding those seats for GA assignment at the airport. These seats are where families without seats together tend to be moved in order to have them all sit together. Some bulkhead seats, usually in EC, are held for special needs (handicapped) passengers, until 24-48 hours before the flight, when they are released.

You should look for seats to select at various elite upgrade windows (starting for DM and PM just after 5 pm ATL time on T-6) and during OLCI. It would be wise to attempt OLCI at around 24 hours before your first flight., when some blocked seats also tend to be released.