The answer is no, by the medieval period in Europe, forests were still rural places, but were largely not unexplored dark lawless places where only brave adventurers would tread.

There were some instances of large wild areas that were largely empty , but in general humans had spread through much of medieval Europe and very much tended to want to keep their surrounding areas relatively safe. People don't really like living in places where they can be victimized by those hiding in the forest next door. Also, depending on the population density of particular areas, even many "deep dark forests" were littered with small towns and various settlements.

You specifically reference the black forest and I'll get to that. But in England, we actually have decent records of forest land. In 1086 William the Conqueror ordered a survey of much of England and Wales. The results of thsi survey eventually became known as the Domesday Book so described because the results were final and inalterable, much like god's judgment on Doomsday.

That book described that in the 11th century, England was only roughly 15% forested. in the years following the Norman conquest the population of England increased dramatically and that number continued to drop as forests were cleared for farmland and woodcutting. By the 13th century the forested percentage had dropped to 4-5%. Incidentally, it's back up to about 13% today.

At the same time, although these two trends are not directly connected. After the norman Conquest, William the Conqurer and his successors often specifically issued decrees that set aside tracts of land and declared them to be "Royal Forests." These forests were protected by laws. As much as 1/3rd of the forested land in southern england was protected Royal Forest. In particular, hunting in the forest was prohibited.

Originally, few things were allowed in the Royal Forest. Commoners could be hassled for merely traveling and accused of poaching. But in 1217 (two years after the Magna Carta) the Charter of the Forest set forth laws to regulate permitted uses of the forest. Commoners were granted the free right to use water and cut wood and hunt inside royal forests. The forest charter actually stood as a statute of England until 1971.

One of these infamous royal forests was "Sherwood forest," which was about 19,000 acres (7800 hectares) in Nottinghamshire. It was about 25% of the land inside the County. But the forest was not uninhabited. There were roads and settlement throughout the forest. It is an area only about 6 miles on a side.

While it's certainly not impossible that bandits could hide inside a 19,000 acre forest. On pathways such a forest can be walked across in a day and ridden across in a few hours.

By Comparison, for US based readers at least, 19,000 acres would make the forest smaller than most US national parks. Virgin Islands National Park is 14,900 acres. Haleakala National Park in Hawaii is 33,200 acres. Yellowstone National park is 2.2 million acres. Grand Canyon National Park is 1.2 million. Yosemite is 761,800.

So at least when we're talking about medieval England, yes, forests existed, but no, they were not generally vast places where the people of that era feared to tread. (Except if they were afraid of being arrested for poaching).

Now, you specifically asked about the Black Forest. Much like the english forests of that era, the Black forest was absolutely littered with small settlements, farms and various agriculture. In Fact, the 1525 German Peasants Revolt began in the southern portion of the Black Forest. It IS quite a bit larger than the english forests, with an area of about 2200 square miles. (6000 sq km) or 1.4 million acres. roughly the size of Grand Canyon National Park, a bit smaller than Yellowstone.

Now, part of the mythos of dark savage forests might actually date back to the Roman era. Because when the Romans occupied the area, they largely did not colonize the forest, but stuck to the outskirts, and the forest itself was largely inhabited by Germanic peoples from the Alemanni. That likely did create a bit of cultural tension, as the Romans would have feared and protected themselves against the Alemanni and Marcomanni living in the forest as part of ongoing roman conflict with Germanic peoples. It's very likely that there would have been occasional raids or low intensity warfare launched from inside the forest.

But by the medieval period, the forest was again, largely populated by farmers and woodcutters and miners who, due to their rural lifestyles, did tend to have their own culture. It might have been a rural place, but it was not a place that people feared to tread because a great many people lived and conducted their lives in the forest every day.

So where does the Myth of the "dark forest" come from? Well, to some extent it's actual folklore that predates the stories of the Brothers Grimm. Germanic and Norse folklore dating back into the premodern era contained the term "Myrkviðr" which is an old norse term for "Dark wood." If you try to pronounce it, you might get a hint of familiarity. It anglicizes into the word "Mirkwood," which was written by William Morris and popularized by JRR Tolkein in his books. The term is used all the way back to the Poetic Edda, and is used in various different contexts to describe forests in Southwestern Germany/Austria, Forests across Northern Ukraine and into Russia in the time of the goths and Huns The Kolmarden Forest in Sweden, and other forests in Southern Sweden.

This is where history verges into prehistory and speculation. But It is somewhat more likely in prehistoric Europe, with a lower population, that forests would have been the kind of places that spawned myths and legends abut what you might find if you walked for days.