I just wanted to post this as I see this every time it's night in Reapers March and I'm sure it's similar in other zones.
You will have a much better chance just riding around from North to South and so on looking. I finally got my bite today and directed others to the spawn location. I was riding around and arrived at an area not marked as a blood fiend known spawn. I was knocked off my mount and to my surprise it was a group of blood fiends in a narrow canyon behind some trees.
I have had this happen multiple times with werewolves too.
When I can pass it on I'll post so I can give out bites.
Don't waste your time camping one area with the other 30+ people. If they aren't there when you show up then they won't be there.