Don't have the phone but after looking at the manual have you tried this.
Then can you highlight the number and is there not a method of turning blocked to allowed.
Look at page 18 : http://www.ukcordless.co.uk/manuals/BT%206510%20User%20Guide.pdf
I have been through the process you describe but the options given are Edit or Delete and all that Edit does is allow a change of number.
So frustrating but thank you for your efforts and response