I presume you are spending a few days in Paris, otherwise the air routing seems odd. I don't think that October is ideal for the Black Forest, but I can't say exactly when the leaves there start to fall. I would point out that hiking is harder with roots and stones hidden by leaves. (I'm not suggesting that the trails are poorly maintained. I'm talking about October.) If hiking (simple, moderate, or strenuous are all available) is important to you, I would rather stay in a town (not Freiburg) which offers KONUS transit cards free with hotel rooms. That makes daytrips easy and free.
It is debatable (!) whether the BF is more attractive or less attractive at the peak of summer tourism. (Ironica typeface.) Because I like urban tourism too (like Freiburg), you might consider Strasbourg as an obvious stop from Paris. Open air daytrips may be do-able from there. It is possible to TGV to Strasbourg directly from CDG if that is what you are actually doing. It is more out-of-the-way, but Basel has some fine museums.
If hiking is not important to you, there are plenty of attractive small towns to visit in Bavaria, as well as elsewhere in Germany. Because you asked, I will say that I stayed at a luxury resort in Hinterzarten, which might be affordable off-season like this. I took the (free) train to Titisee (mass tourism destination) and walked back to Hinterzarten. Another day I trained to Freiburg for the day. Parkhotel Adler is an easy walk from the train.