Steve
Most excellent question and one to which I have yet to see an answer or for myself to fathom. All iron blue DUNS i have seen have an more orange than red colouration around the head. the red colouration is more in keeping with the jenny spinner for which the Dun patterns are a very poor imitation anyway.
http://www.riverflies.org/index/rive...ron%20Blue.pdf
Like yourself I am confused by this apparent annomaly and would love to see an explanation. Sounds to me its one of those situations where a pattern has taken fish over a long period of time and has been handed down without being really questioned. The scarcity and unpreditability of Iron Blues on many rivers may well partially explain this - but that is pure surmise on my part !!!