Thanks for your help. Unfortunately, the cast() did not work. There are not so many useful search results on Google on that topic. Seems that not so many people try to handle blob data from the cli. In the end, I found a solution that works:
Code:
sqlite3 ~/Library/Caches/com.omnigroup.OmniFocus/OmniFocusDatabase2 "SELECT QUOTE(valueData) FROM Perspective ..." | sed -e "s/^X'//" -e "s/'$//" | xxd -r -p > test.plist