blue on blue
Describing an instance of accidental violence between members of the same group. Primarily heard in UK. I don't think Joe was shot by our enemies—I think it was blue on blue.
blue on blue
used 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.