Which could be it abating - or merely getting lost in the slipstream. That it's visible at all at M-way speeds implies there's quite a bit of smoke.Ermm....well...need to be a little careful here...
Lets just say that when I give it the boot at 'motorway speed' that is when I notice it. If I keep my foot down it fades off. And it definitely appears to be black, not blue.