Is a hematoma from a bruise dangerous? could this be a source of dangerous blood clots or something similar?

No: Bruises (hematoma) will change colors and go away slowly. You may get a scar lump due to vascular bleeding and healing. It is not painful. There is no treatment for contusions, bruises, hematomas. They may look very ugly but are not dangerous. Whatever causes the bruise needs to be considered. If you can prevent another one best to use common sense and try to stay away from the problem.