What is the neatest way to run power to Somfy motorized window blinds?
In the end, the best way to solve this was to request replacement Somfy motors with longer cords. The factory can supply them at any length, and they arrived within a week.

I have now had them installed and there was no junction box needed, since there is no join in the cable behind the wall. There is now no visible cable whatsoever.

I hope this is helpful to others who run into the same issue.