KPI’s for blogs have not been defined yet (as far as I know). Here my approach to turn web analytics data in something useful:
Top 10 KPI’s for blog
1. Average visitors per blog post (new/returning…)
2. Average comments per post (per day/week/month)
3. Number of new commentators per day/week/month
4. Number of returning commentators day/week/month
5. Everything related with RSS (new subscribers…)
6. Single access pages (drop out rate / bounce rate…)
7. Number of searches per day/week/month
8. Number of trackbacks per day/week/month
9. Number of Akismet spam per day/week/month
10. Number of Incoming links 
Feel free to add/comment!
