Blue skin for special powers?

[mwf]

I'm trying to remember a sci-fi book I read when I was very young. I can remember only a few details: the protagonists lived in a middle-ages-ish level of technology, but there were starships in orbit. The most distinctive feature I remember is that there were some kind of symbionts which gave special powers of healing and longevity - something you "installed" by cutting your skin and placing them in the wound, and then they grew all over? They became your skin, I think. It's possible that they could be passed on - removed from one person and given to another. There may also have been a female character named Fava - but I could be wrong.

Anyone have any idea?

Here's hoping it's a good book - it's been stuck in the corner of my brain for about twenty years.