Auto buyers and sellers often use Blue Book values as a starting point when negotiating the price of new and used vehicles. Though widely viewed as generic synonyms for a vehicle's market value, the terms "Blue Book" and "Blue Book value" are actually registered trademarks belonging to the Kelly Blue Book Company. The California firm's print and online Blue Book guides are the leading sources of information on the values of new and used vehicles.
http://www.kbb.com/
eHow Article: About Blue Book Values