>How would you insure that the entries in a given field are unique OR blank? That is, multiple blank records are OK but if data is entered, it must be a unique value. Setting up a candidate index doesn't work due to the blank fields.

Filter the candidate index:

CREATE CURSOR a (b i)
INDEX on b FOR NOT EMPTY(b) TAG OnceNoZero CANDIDATE
ON ERROR wait window MESSAGE(LINE(1))

INSERT INTO a VALUES (0)
INSERT INTO a VALUES (0) && no error
INSERT INTO a VALUES (1)
INSERT INTO a VALUES (2)
INSERT INTO a VALUES (1) && error