The County of Blois is originally centered on Blois, south of Paris, France.

One of the chief cities, along with Blois itself, is Chartres.The extent of the county varies over time.

The northern portion, bordering on Normandy, is sometimes alienated as the County of Chartres, but the Counts of Blois who possess it do not use a separate title for it.

These lands are finally sold to the crown by Joan, Countess of Blois in 1291.

In 1439, the area around Châteaudun is separated as Dunois for Jean de Dunois, appointed Count of Dunois.