BlackBerry's BBM messaging app for Windows Phone is currently not available to download in the Windows Store. There's no word on why this has happened.
The Windows Phone version of BBM first launched in 2014, after a closed beta period. It's possible that the app's disappearance is a glitch on the Windows Store side. We have contacted BlackBerry to see if they have removed the app from the Windows Store, and if so for what reason, or if there is an issue on Microsoft's side.
Thanks to Moustafa for the tip!