When you've become deep enough friends with someone, that you become unaware of their flaws and imperfections. Someone with friend blindness would stand up for their ugly friends, telling everyone how hot he/she is.
When you think your friend is a 9, and everyone else thinks he/she is a 6.
Terrie: "I remember when I met Alexis, I thought she was so average. But after knowing her for awhile now, she has really become quite beautiful."
Chris: "She looks the same as when you met her. You suffer from Friend Blindness."