If you stay outside the main cities there is a scheme called Konus Card, which is a free travel on all public transport including regional trains, supplied by the hotel to cover your full stay.

https://bw.tourismusnetzwerk.info/wp-content/uploads/2019/05/KONUS-Flyer-PDF-Download_GB.pdf

Although our plans changed, we planned to stay in Donauschingen - the source of the Danube.

 https://www.blackforestgermany.com/donaueschingen.php

I can recommend Freiburg (am Breisgau). Not supersmall (about 250 000 inhabitants), but a cute city with a nice historical centre and quite eco-minded. A lot of things to see and do, nice restaurants and bars and easy to acces the black forest by train (to Donaueschingen e.g.).

You can also access some walks easily with the city’s tram network.

I really loved it, two years ago :)

Staying in FB gives you NO konus.

Best known smaller town is Hausach, with the world biggest cuckoo-clock. However, this sits along the ´real ´Schwarzwaldbahn wich currently has severe restrictions.

Better towns are spread out along the line going east from FB: Hinterzarten, Kirchzarten, Or the much more touristy Seebrugg/Titisee (yes, on a ´see´=lake).

Via that Konus-site you can click on t find listings of nearly all accomodations in the participating towns.