If you are close enough to communicate via Bluetooth, can you use VoSW (Voice over Sound Waves)? 
What are the kind of use cases where one would want to do chat messages over Bluetooth? i.e. where just using a chat app over the internet is precluded?
Maybe you want to send/receive a file? (and no internet is available)
Dragging something in from another current topic maybe you could use ad hoc WiFi networking / WiFi Direct to achieve this over WiFi rather than over Bluetooth i.e. better bandwidth and more accessible protocols and APIs. So another question would be: what would Bluetooth give you that WiFi doesn’t? I would think that just about any current or recent smartphone would support both WiFi and Bluetooth.