I had the exact same problem, Apple and American Oxford Dictionaries became unavailable with empty blue circles and I could not re-download them following upgrade to IOS8. Spent an hour and a half on apple support chat last night with no fix. Tried everything everyone on the internet suggested over the last few days to no avail, but I finally figured out the fix.
1. Open itunes.
2. Choose library, apps and list.
3. Right click iBooks in the list and choose "Show in Itunes Store."
4. Once your on the iBooks Itunes Store page, choose "update."
5. After the iBooks update downloads, connect iPad to computer, make sure iBooks on the iPad is closed, and click choose, File, devices, sync iPad.
6. After sync completes, eject iPad.
7. Open iBooks on iPad, highlight a word, click define, manage and previous dictionaries should be back and functional.
At least this is what worked for me.