See this ⌘AskDifferent article on what to do if your plist array property has one element, or more, and the different approaches to removal of just one, or the nth element of the array. The defaults command does not provide you with the granularity to remove the nth element of an array, so first check how many elements are in that plist array:
defaults read /Library/Preferences/com.apple.Bluetooth.plist IgnoredDevices
If there is more than 1 array element in IgnoredDevices array, then you will need to use Apple's PlistBuddy to remove the array item. These are zero-based arrays, so in the following, let n be that array element:
sudo /usr/libexec/PlistBuddy -c "delete :IgnoredDevices:n" /Library/Preferences/com.apple.Bluetooth.plist
You may need to reboot after you fix this IgnoredDevices entry.