White, generally, a real speed. Trainer speed, gradient, cadence, calculated power and registered weight agree in bkool's formula to a real world speed.
Blue not so much. Blue is bkool's best guess when one or more of the above don't fit within the formula.
Some have written about beating the blue (sounds rude) on downhills. The weight I am means I'd be very hard pressed to do that on anything above (below?) maybe -2% pushing out less than 4 or 500w or more. If you look at the descenso sessions I do as a warm down, -5% or steeper down gradient, 60 or lower cadence and 20-40w gives me a solid blue speed of 50+ mph (wheel speed on the garmin edge generally <5mph). The fastest I've seen was very late 60's.
I just use these sessions as a keep the legs moving to ease them to a resting state.
Thank you for the detailed explanation. I had no idea.