Another spelling of Blais also is Blaise, commonly Blaize in areas of France who speak Franco-Provençale (also called Valdotain, Valasien, and Forézienne), especially Savoy, France and Val d'Aosta, Italy which has about 70,000 speakers according to recent European Community reports. (Some people also speak this language in the canton of Vaud, Suisse/Switzerland and a few in the départements of Jura, Doubs, Ain, and Isère, France.)
(The native language of my family is Franco-Provençale. It is neither French nor Provençal, but a completely different, ancient, Latin language. Many words in French derive from it. In English, ordinary words of this origin you might know include « chalet » and « avalanche ».)
Other Franco-Provençale names include: Charvet and Charvoz (in French, Chauvé, Chauvin, and Chauvot; in Latin Charveys; all from meaning bald or treeless in English), Thollon (surname variation for French name Antoine), Berlioz, Chartraz, etc. (As you can see by my selected examples, typical names from Franco-Provençale many times have the silent letters z and x, the sound of « arv » where one might expect « auv », and also double letter LL.)
Back to the name Blais, which if pronounced « BLAI », no S sound (in English, I think this is not likely), it could come from the French word for wheat, which is « blé », but this is only a slight possibility.
I hope I did not make too many mistakes with my English for you in this message.