does the house that you're staying in have wifi that you can use? if so you can pretty much ignore the whole sim card and use apps like Whatsapp or Viber to send messages and call friends family completely free. Failing that you can buy pay-as-you-go/top-up type SIM cards but i'd go for the local card (so french in your case) as otherwise you'll pay roaming fees which cost about a billion times what the normal local rates are (i am ony slightly exaggerating here :P  ). In switzerland you can't get a contract SIM without a certain permit (L permits you need to wait 3 months) so the top-up SIM would be your best bet if you get a swiss one. not sure if you can get a contract SIM in france easily or not. just be aware that as soon as you go roaming all phone, sms and data costs prices drastically increase. 

If your Blackberry is "locked" to the original Italian carrier then you won't be able to simply drop in a new SIM from a different company.

Which model is it?

in Cessy, you might be able to use a Swiss SIM from Sunrise, without roaming. They provide phone services for CERN, and since there's a CERN site in Cessy, you may be able to use that antenna.