If you're trying the cross country mission, there's a couple of extra difficuties. First of all, i don't know if this is how it spawns in all missions, but in that one the Blenheim spawns with quite a bit of right rudder trim dialed in. There is a spring-loaded bar indicator on the bottom right of the instrument panel with the neutral setting clearly marked, dial in some left rudder trim until you get it to neutral and then some.
Most of all, in the cross country mission there's a significant crosswind from your right to deal with on top of the already existing pull to the right.
It's very visible if you try that same mission with the Tiger Moth which is lighter and easier to observe the effect of wind on. First it veers to the right with the engine still off, then as you apply power, start rolling down the runway and commence your take-off run you can see it's dipping a wing in accordance with the crosswind.
So it's not only the Blenheim's engines pulling right, it's the trim too and the crosswind. Out of all these the trim is the easier to correct. The pull by the props could be mitigated by mapping keys to select the engines separately and give more power to the right engine so that it wants to turn left and counteract all these effects. However, the crosswind is built into the mission's weather conditions.
I get the feeling that the default setting of "blenheim+crosswind that pulls the same way as the props" is meant as some sort of challenge. If you open up that mission in the FMB, remove the wind (set it to zero) and save it under a different name, i guess you'll have a much easier time
