We will be docking in Breisach in early April for a single day, and would like to visit Colmar (where we haven't been) and also see some Black Forest scenery plus visit Freiborg (where we HAVE been before). We could do these things on tours given by our cruise, but would like the flexibility to go when and where we want without sitting through the things we aren't interested in during the group tours. Does anyone have any suggestions for private local guides to contact?

Unlikely anyone here knows but I suggest you contact the Breisach Tourist Office with that query.

Breisach (Germany) - Colmar (France): bus every hr, journey time 50 min.
If you leave Breisach at 8.56 you will arrive at Colmar at 9.46. The exploration of Colmar will take at least 3 hrs and lunch another hr.
If you leave Colmar at 14.05, you will arrive at Freiburg at 15.34. There may not remain much time for Black Forest villages.
I would rather spend the afternoon in the vineyards of Riquewihr and Ribeauvillé, both close to Colmar.

Yup, that's a totally unrealistic plan. Also notice that Breisach while heavily bombed in WWII has an absolutely outstanding church, the St. Stephan minster. It has the huge Schongauer frescoes, the silver reliquary, the choir screen and especially the carved altar which is one of the great masterpieces of its art.
That the extensive German wikipedia article https://de.wikipedia.org/wiki/Breisa...nsm%C3%BCnster has no counterpart is a shame ...

Our cruise includes a 4 hour morning tour that drives through the Black Forest and stops for "clockmaking" and visits Freiburg, then offers an optional (add-on expense) tour in the afternoon to Colmar. Since we toured the Black Forest area in our own car on our own time a few years ago, that's why I figured having a private guide and car for the day would be better - then we can see Colmar on our own schedule, and only revisit Freiburg on our own pace, since we loved it, not wasting time with the group. Of course a local guide could also show us the highlights of Breisach! I am going to pursue this thru the Breisach Tourist office as suggested above, but if this alters any of your suggestions, let me know! My understanding is that Freiburg and Colmar are both only 30 minutes or so from Breisach by car, but in opposite directions.

Highlights of Breisach?