blue or black

I went shopping yesterday and got these really comfortable blue khaki pants. Once I got home and tried them on they looked black. I was so confused because at the store it wasn't even a question, they were defiantly blue. I put them up against a blue shirt, they were black. Up against a black shirt, they were blue. So I wore them today not knowing if they are black or blue, wearing a white shirt to play it safe. So what is the real test to tell what color they really are?