I have 2 honorary uncles from my childhood. They've been best friends with my dad since they were kids. One day my brother was doing a family tree for school and he got a confused look on his face... "Who is Uncle Dave a brother to?"
Uncle Dave is black. We're European mix/Cherokee.
I have a few friends who have the title "Aunt" or "Uncle". I am the "Aunt" to a beautiful newborn baby boy.
My kids don't see their biological aunts and uncles, unfortunately. Mostly it's due to physical distance. I have a biological nephew who lives on the other side of the world (Philippines).
Right now, I'm debating what to call my future step siblings. My dad is getting married and his future wife has 3 adult children. I think it would be appropriate to refer to them as Aunts and Uncle. However, I haven't met them. Scratch that, I went to middle school with the boy. I don't think we'll refer to them that often, though, so I think it's a non-issue.