Hi BJS,
You did'nt say if the urn is in sun or shade, so I've selected a few that will also cope with some light shade as well as full sun.
Gentian (Gentiana acaulis) has blue trumpet-shaped flowers and a mat of dark evergreen leaves. Probably the most vivid blue of all!
Arenaria or sandwort should also trail down nicely, it's leaves are also evergreen and the flowers are small and white.
Campanula (Bell flower) I think may be a good choice, the flowers can vary from dark purple to lillac or white and are easy to grow. Most varieties will thrive in sun or shade, C. Compacta is a nice one. Good Luck Tbird+