In the Blenheim I regulary bomb shipping.
My favourate loadout is the "4XGP Bomb, 250lb., MKIV."
If you are bombing from low level make sure you set the fuse to the "No28. IIx Pistol Everready". Remember to set it to 11 seconds or you will have a hard time.
If you are skip bombing get as close to 90 degrees to the side of the ship as posible, otherwise the bombs can (and will) ricochett of the side of the ship.
Against tankers it is not uncommon to get 4 ships out of 4 bombs.
In the Geman bombers, make sure you arm the bombs before dropping them otherwise they will not go off.
If you're into multiplayer get on to ATAG with teamspeak, It's a hoot when you can get a flight of bombers together!
One of my gripes about bombing in CoD is that there is no sound or other indication that it has left the plane when you press the tit.
Cheers!