Matching artists names in iTunes
The easiest way i can think of : 


Quit iTunes
Get a Tagger (MusicBrainz Picard, Tagger, Metadatics,...)
Go To your ~/Musik/iTunes/iTunes Media/Music folder
Drop all the "James Blunt" Folders onto the Tagger
Set the Artist to "James Blunt"
Save the Files


After you have done that, reopen iTunes.

Now you have 2 Options :
You can either go through the Tracks by yourself and start listening so iTunes sees, that you changed the Artist.

Or, you can use an AppleScript to refresh your tags.

Your Music should now be cleaned.

And yes, that is in my opinion the easiest, you can go easier, but that requires more scripting which therefore would not be easy.