Proper noun
Originally, the flag of the Rear Admiral's squadron in the English fleet.
The flag flown by a British naval auxiliary vessel. Members of certain British yacht clubs may also fly a blue ensign if it has been defaced by the inclusion of an insignia in the fly.
Source: en.wiktionary.orgSpey will enter Portsmouth under the Blue Ensign which is denoting a vessel in government service, rather than one in the Fleet. Source: Internet