It depends on the aircraft and the route (domestic vs international). In general, DL blocks the first half-row of C+ (2-4 seats), and the last 1-2 rows of normal economy (6-12 seats). Of course preferred seats occupy the isle and window toward the front of the plane, and you can see this on DL's aircraft page.
Using a tool I'm working on, here is an example with the B's being blocked/hidden on the normal seat map for a 738.
Code:
FLIFO: DL877, IND, LAX, 11Nov
O-Open X-Occupied B-Blocked
Row: A B C D E F G...
1: O O O O
2: O O O O
3: O O O O
4: O O O O
10: O B B O B B
11: O O O O O O
12: O O O O O O
13: O O O O O O
14: O O O O O O
15: O O O O O O
16: O O O O O O
17: O O O O O O
18: O O O O O O
19: O O O O O O
20: O O O O O O
21: O O O O O O
22: O O O O O O
23: O O O O O O
24: O O O O O O
25: O O O O O O
26: O O O O O O
27: O O O O O O
28: O O O O O O
29: O O O O O O
30: O O O O O O
31: O O O O O O
32: B B B B B B
33: B B B B B B