Our Gothic cathedral was raised on the foundation of a small oratory dedicated to saint Pierre, then dedicated to Solenne, when they brought the saint's relics in 589. Solenne? Oh, yes, he was bishop of Chartres in the 5th century. The legend said he was there with saint Rémi for Clovis' baptism... He died near Luynes castle, in Touraine. His successor sent his henchmen in order to bring back the corpse and bury him in Chartres.
The thing was done in three days: henchmen came back in Chartres, but before, they made a break in Blois. Here, inhabitants offered to keep Solenne's relics for the night, in their church... On the next morning, when henchmen wanted to leave again... they were incapable of lifting up the shrine! The legend said Solenne appeared to the bishop of Chartres and ordered him to leave his relics in Blois!
Anyway, they rebuilt the church in 650 and in 1016 (a kind of Romanesque-Byzantine basilica), then count of Blois Thibault the Great completely re-raised it one century later! The building works were completed in 1106. The cathedral was damaged, especially during the war of Religion, in 1568: Protestants even burnt Solenne's relics!
At the end of the 17th century, a storm damaged our church: the bell-tower only remained after that... Marie Charron, Colbert's wife (Louis XIV's famous Finance Inspector) asked the king to rebuild the church. It was dedicated in 1730 with its current name: saint Louis, in tribute to the king! The church was consecrated cathedral in 1697, when the bishopric was created...