This is one of my favorite castles in France, and need to be seen more ,therefore, took it out from my older previous post and repost the links here.
https://paris1972-versailles2003.com/2011/03/16/blois-royal-and-magical-and-houdin/
https://paris1972-versailles2003.com/2015/12/25/the-magic-and-architecture-richness-of-blois/
Enjoy Blois , plenty of info inside the posts above. By car is about 1h30 from Paris . Get on the A10 direction Bordeaux and go out on exit/sortie Blois, about 170 km or 105 miles or go on the no tolls road N152 direction Blois-Orleans for about 180 km or 112 miles. If no GPS in the car use viamichelin for the routing online. By train from Gare d’Austerlitz Paris to the station of Blois-Chambord in about 1h40 or from Gare Montparnasse Paris on TGV to Vendome and then to Blois by taxi 30 km or to gare Saint-Pierre des Corps just outside Tours and then 30 minutes on taxi to Blois.
By bus OuiBus takes you from Roissy CDG or Gare de Bercy in Paris here: https://www.ouibus.com/
or the Flixbus (former ) here: https://www.flixbus.co.uk/
Enjoy the ride and a wonderful castle ,not to mention very nice city to walk and see wonderful heritage of France. Cheers and enjoy your weekend wherever you are.