Spent a couple of days in the Black Forest in May a few years ago. While we were there it was pleasantly warm during the day. The roads were all clear, although on the highest roads there was still fairly deep snow elsewhere and it was not the place for summer gloves.
We camped for a night near Freudenstadt. At eight in the evening as we walked to the restaurant, it was a little fresh. Walking back to the tents around 11:00 it was properly chilly. Woke up at 3 am and it was absolutely bloody freezing! Struggled to get back to sleep as it got colder and colder, and got up around 5.30 just to stomp around abit. Ice on the tent - not just a light covering of frost, but proper ice. My friend emerged from his tent wearing all his clothes, including leathers, helmet and gloves, and said he was still too cold to sleep in his sleeping bag.
As soon as the sun climbed over the mountains around 7:00, it was instantly warm again.
The Black Forest has some great roads - in fact, pretty much all of them are great - and scenery. There are some serious mountain passes, some big and sweepy, others small and technical, but all good fun. You'll love it, as long as you're not a poorly-prepared camper. Titisee is a bit of a tourist trap, full of coach trips on the day we visited. There are many other nicer places around, just don't ask me to name them!
(Haven't been to Millau in May, but I imagine it will be significantly warmer and sunnier that the UK.)
