Ok I’ve blown the dust off my project and discovered the following things:
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If you connect using Adafruit’s Bluefruit LE Connect App to Adafruit’s example code it will offer to update the BLE firmware on your feather board. Not sure if this is important or not, release notes can be found here..
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Something has changed in the BLE / Blynk world that means you can’t initialise Blynk until you have fininshed initialising BLE. This means the code in my previous post no longer works, specifically you need to reorder these lines:
ble.begin(true); //true => debug on, you can see bluetooth in the serial monitor.
ble.factoryReset(); //Optional
ble.setMode(BLUEFRUIT_MODE_DATA);
Blynk.begin(auth, ble);
The bad news is that I’m still seeing odd connection issues - I’m able to send commands from the blynk app to the feather board, but I’m unable to transmit in the other direction 
. The app logs messages being sent from the device to the app, but the app doesn’t update. Worse, the board see heartbeat timeouts (though even though the app believes the remote end is disconnected it still receives commands!). I’ll continue my educated guesswork and see what I can find…