What does "blue-on-blue" means?

[waize]

I red about in a issue of Combat & Survival, but I did not get any full explanation of what it means.
Please help.

[Ichhabe]

a better phrase for Friendly fire :|

[Chet Mystery]

Blue = good guys

Red = bad guys

If you accidently attack your own team, thats called a blue-on-blue engagment (aka, a major ****-up)