The hardware business is dead and dusted. Amazon needs a platform to sell you things, not a difficult hardware business. Blackberry's only useful assets are their enterprise software and sales teams; everything else is noise.
Depending on where Amazon wants to take Kindle, that could be useful.
It's a natural fit and like I mentioned above Amazon was quite interested 18 months ago, I doubt much there has changed except the fact that the market cap for BBRY has fallen since then, making it an even more attractive asset to Amazon.