The Blenheim needs full left rudder and some left rudder trim to track straight until the tail comes up in any mission, whereupon you need to progressively reduce the left rudder as speed increases. Applying power gradually rather than suddenly, and dabbing the brake with full left rudder applied, also helps control the torque.
The reason for this is that the Blenheim's engines both spin in the same direction, which produces a lot of torque reaction, pressing the starboard undercarriage wheel into the ground and hence forcing the aircraft to the right until airspeed is reached. Even then, it still needs quite a bit of left rudder trim in the air in order to keep the 'slip indicator' needle centralised.
The effect of this torque reaction is exaggerated in the stock 'Cross Country Quick Mission' due to the presence of the wind and full fuel load.
If you're able to join the ATAG server online, selecting 50% fuel load or less and following the video guide, you should be fine even with all realism settings on (which is fixed by the server). Also if you download and install Teamspeak 3, there are a lot of the guys flying Blenheims who'd be happy to give you some one to one advice and guidance.
Hope you get chance to give this a try.
