BlackBerry hasn't really kept the fact that BBM is coming to Windows Phone a secret. In fact, they've made several announcements on the matter with the most recent noting Windows Phone users would indeed be able to download the app at some point in 'July'. Of course, some beta testing has to occur before a roll out of the app happens and it seems that's possibly already underway as BBM has been spotted sitting there in the Windows Phone store with compatibility for Windows Phone 8 and Windows Phone 8.1.
Update - Link has now been confirmed as real and there will be more details on how Windows Phone users can take part in an external beta coming up soon via the BBM Blog.
Sadly, it's not confirmed to be legit even though it was added by BlackBerry Limited (the name BlackBerry uses for all apps published) and given it's marked as beta, the download access has also been restricted meaning only those with invites can download and use the app. There's no way to sign up for access either, in case you were wondering.
Windows Phone Central reported earlier that Windows Phone UI elements would be present, while preserving the familiar BBM design but again, as it's listed as a beta BlackBerry was smart to hide any screenshots of the app so as to not reveal too much of the Windows Phone version before they were good and ready. Know anyone on Windows Phone who will be looking to download BBM as soon as it's available? Seems some folks at WP Central are excited.