Blue is the Colour...

... of a number of round plaques found across Britain.  They are markers indicating a location of historical significance, or more usually, denoting a link between a particular location, and a noted person who lived or worked there.  These plaques, often erected by English Heritage, but also by other various organisations, can refer to individuals from all walks of life.
Here are a few examples :

Robert Graves (Writer and Poet)

London SW19

Robert Graves, Blue Plaque by Jessica Mulley is licensed under CC BY NC ND 2.0

Josephine Butler (Women's Rights Campaigner)

London SW19

Blue Plaque on the home of Josephine Butler by Jessica Mulley is licensed under CC BY NC ND 2.0

Peter Cushing OBE (Actor)

Whitstable, Kent

Peter Cushing O.B.E. Blue Plaque by Paul is licensed under CC BY 2.0

London to Bath Coach Road

Calne, Wiltshire

London to Bath Coach Road blue plaque by sleepymyf is licensed under CC BY NC ND 2.0

Captain Noel Chavasse VC and Bar (Soldier)

Liverpool

English Heritage Blue Plaque - Noel Chavasse by SteHLiverpool is licensed under CC BY NC ND 2.0