Not quite sure about that. Around 2009/2010 when I lived in Cambridgeshire (can’t recall which year) a significant proportion of a bumper second brood were the blue females. As a parallel, in some of the great years in the 1970s, valesina female SW Fritillaries were numerous.Another blue female Common Blue too - I think there are more in the first brood than the second, and this may possibly be linked to cooler temperatures when in the pupa. This sort of temperature variation affects the colours of quite a few species - maybe this one too.
My hypothesis is that blue female Common Blues, valesina SW Frits and other unusual forms are always present in the gene pool but the early stages are less robust and suffer higher mortality and only survive to adults in the most favourable conditions.
If my hypothesis has any validity then this could be tested by captive breeding where mortality of early stages should be less than in the wild. Blue female Common Blues might be in be proportionally more numerous regardless of temperature in the bred stock.
Jack