blue on blue
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blue on blueused to denote an (inadvertent) attack by a military force on members of its own side. British
The expression derives from the use of blue on maps to designate one's own forces.
2004 BBC Press Release If I had been told on the day that Christopher died that it had been blue on blue, I could have coped with that, things do happen in war, mistakes are made, casualties, it happens.