I think Mendix will not integrate that fast Siemens bluetooth devices into Mendix :D
Anyway, your bluetooth device should talk to your Mendix app over internet. So get a bluetooth device which connects to wifi and has a service behind it to connect with or so.
Or your mx app should detect through a bluetooth device the other bluetooth device and talk over bluetooth, seen this seems unpractical (server is somewhere in the cloud) I would consider option 1.
What kind of data are you getting from a bluetooth device, is there no sensor with internet connection? (see demo with sensor integrations of Johan den Haan at mendix world 2016)
At the moment there are no widgets in the app store that will bring Bluetooth functionality to your app.
If you need the function you could deploying your mobile app via phonegap and build a widget.
You can build your own widget with one of the plugins https://www.npmjs.com/search?q=phonegap%20bluetooth
Please note that there is no such thing as a generic "bluetooth" solution. As bluetooth a trasnport method and the specs are huge and hardware suppliers will build there own communication based on top of the transport protocol.
using widget only,
gulp build (make sure that you have gulp cli)
get mpk file from /dist/...mpk