on Mac, you're going to need to use your IAC Driver Bus to point your MIDI to, not the Focusrite. i wonder why it's not recognizing that as an available source. what does your actual startup script look like?
On the former laptop (MacbookPro 10.13. with JackRouter) I didnt`t configurate anything inside of Audio-Midi-Setup besides the following lines in SuperCollider and it worked properly:
Now I failed and tried to activate all MiDi-Parameters of IAC-Driver inside of the Audio-Midi-Setup, but still the Focusrite is not recognized. I don`t know.
i got a new laptop (w/ Catalina) and a new interface (UA Apollo) and started using Blackhole recently and i remember this being a pain to set up again. trying to remember what all i did. have you set your SuperDirt startup settings to include all 16 virtual channels for Blackhole yet? after which you might want to restart.