The ‘Blue Spitfire’ which flew at the Malta Celebrations was BM597 MkVb belonging to the Historic Aircraft Collection and kept at Duxford.
It was painted in U.S.Navy Blue to represent U2 (one of 18 Spitfires on board) which was flown off USS Wasp, for delivery to Malta.

Thanks for the info bgrose, I don’t think many of us were aware of the facts.