The "bling" you hear before take off is to let the cabin crew know that UNDER NO CIRCUMSTANCES should they get out of their seats as the aircraft has turned or is about to turn onto the runway and takeoff is imminent. After take off, it is as stated by other contributors, to advise the cabin crew that they (but not the passengers) can get out of their seats. Hope this helps.
As far as I know, all electronic devices can be used after the seatbelt sign is switched off after take off (usually about 18,000 feet) until it is switched on again during the descent (usually about 15,000 feet).
Hope this helps.