I use blackhole on my M1 MacBook Air and it works just fine.
Did you:

  • Go into Audio Midi Setup
  • Create Multi-Output Device
  • Set Macbook Speakers as master device
  • Add on any other devices you want to hear audio from (for your headphones)
  • Go into Sound Settings and set Multi-Output Device as your output devince
  • Go into OBS Audio settings
  • Set Desktop Audio to Blackhole.