I have deleted Blueprints associated with a pick-list field, yet I am unable to edit the pick-list values.
If you wish to edit the values of a pick-list field while you have Blueprints based on it, you will not be able to edit the values until the Blueprint is deleted.
Even after deleting the Blueprint, if you continue to get the error, it means you have records that are part of the Blueprint you just deleted. Either the records must exit the Blueprint, or the records must also be deleted in order for you to edit the pick-list field associated to that Blueprint