Getting audio from an external USB mic into OBS

[BG213]

I am trying to get both audio from an app and an external USB headphone-mic into OBS. The audio from the app can be brought in just fine via Blackhole but when I plug in either a USB external mic-headphone or another one I have which connects via bluetooth it records that mic but not the Blackhole-sourced app audio! Disconnecting the external mic-headphones then restores Blackhole-routed audio. Clicking in the external audio input source into Blackhole has this effect as does having OBS separately capture Blackhole as an output audio source and the external mic-headphones as an input audio source. Basically Blackhole and any external mic conflict! The only working mic I can use with Blackhole-sourced app audio is the system mic but that is pretty bad (low + plenty of keyboard sounds). What can I do? Some googling doesn't find a place to really ask questions about Blackhole so I turned here. Thanks in advance if you have any advice!