RENURRP,
Thanks for the reply, and thanks for trying to actually answer the question.
You have got me thinking that the reason it is marked, is to give you a speed to fly should you lose one in cruise. If you are going down, then this speed will give you the most amount of time remaining in the air.
If you are going down into the sea or jungle then that extra time could be very important.
To really find the answer to this question, you would need to delve into the history of multi engine flight/certification. Perhaps it was during the early days of big radial engines, when engine failures were much more common, that someone had the bright idea of trying to maximise the remaining air time.
Maybe it has some origin during wartime, when the extra time aloft could make the difference between whether the occupants all manage to bail out or not.
I am wondering this about blue line because I suspect that modern twin training is giving this speed undeserved emphasis - to the point where pilots single mindedly plan this speed without any thought to vxse (I know a couple of pilots that believe that, if you are going to climb, it will be at blue line but not a knot below).
I will try to research further and post my findings back to this forum.