From what I saw in BlackBerry World, it is for BlackBerry 10 devices. To be sure, from your device, go to BlackBerry World and search for the app. If it appears with the download option, it should work. Another option would be do go to the vendor's website.
SayIt is for BlackBerry 10 devices only. There are a few voice command apps for BlackBerry 7 and earlier devices, but they are not as full featured as SayIt. IMO SayIt isn't as good as iPhone's Siri, but it comes pretty darn close.