I used Codename One, another app development platform that you write most of the code in Java and then can build native app files for Android, iOS, etc from one code base. It wasn't that difficult. In CN1, I did have to write the native code for the BLE part, but I found some good examples for each platform. Then in CN1 you can link it all together so it builds it into the app. I'm not a professional developer, and didn't know much when I started (not even Java), but it wasn't too hard.
I'm pretty sure Xamarin users have created libraries for BLE as well.