to contend that 'blue line speed' is the defining speed
Yes, it is simplistic (and that is one of the attractions) in the same way that "reject below V1"/"continue above V1" is simplistic. Clearly there will be quite some variation in the application of such a decision process according to the Type .. but one needs to start somewhere in managing an engine failure without too great a need for contemplation at the time of the event.
As an aside, from my own history with light twin IFR renewals, I always used precisely this concept without ever having an examiner query my logic .. but then they all knew that, if they pulled an engine earlier, ... the other would follow by my hand .. this was made easier by the fact that I rarely was faced with taking a twin out of a short strip.
For a short strip departure, however, my philosophy would be altered to suit what I assessed to be a reasonable middle ground of risk management. Certainly for typical shorter field twin operations, if the operation is to occur, one has to lift off earlier than blue line and just accept a higher risk period until the aircraft is cleaned up and climbing away at a suitable speed. In general the greatest risk was driving out to the airport so it all needs to be kept in perspective.
The reality with light twins is that the takeoff is divided into three reasonably distinct phases .. low speed where one has no choice but to stop as best as one can arrange .. high speed where one may/should continue subject to OEI climb capability and surrounding terrain (and, perhaps, weather) on the day .. and a very BIG grey area in the middle which could take up a lot of time in discussion over a wine or three ...
More often than not on heavy Quads. the TOSS is several thousand feet
We all need to keep firmly in mind the reality that "light" and "heavy" (aircraft) are related in a similar fashion to "black" and "white".
The FAR 25 animal is characterised by comparatively high levels of redundancy and safety (ie low risk) while the simplest FAR 23 version is at the other end of the spectrum .. all very reasonable when one considers the target market, usage, and cost/benefit aspects, etc.
Point is one needs to be very careful when trying to compare apples to oranges .. trying to overlay Part 25 philosophy onto the Part 23 aircraft is fraught with difficulty.