Many iPad sequencer apps, particularly the older ones, do not transmit MIDI to external devices, but rather hardwire it, internally, to their own, and usually limited, sound module circuits, but you will normally not find that prominently listed in their spec sheets or owners manuals. This has been the major criticism of Garageband for the iPad, since its inception. I would say that you need to do your homework, but with inaccurate and/or limited-info spec sheets, that is not aways possible. I have found that many of the legitimate app writers have their own websites, so if you go there, you can normally get a lot more info than just reading the hype in the iTunes App Store, but no guarantees on that, either. If the app, itself, does not transmit MIDI externally, to begin with, then Bluetooth will not help, and even if the app DOES transmit MIDI externally, it would also need to be Bluetooth enabled.