Most terminal applications use the $EDITOR environment variable to decide which editor to use. Set it to blackbox with environment.variables. Careful though; since it’s a gui-only editor, it will not work in non-graphical environments.
As for gnome, you’ll need to make sure it has a .desktop file in the package, make sure it registers an appropriate mime type, and then set it as the default handler for that mime type using xdg.mime.defaultApplications.
Erm, sorry, you meant terminal emulator o\ That’s up to gnome settings, I have little experience with those. Gnome is notoriously hard to configure declaratively, it will probably involve services.xserver.desktopManager.gnome.extraGSettingsOverrides.