The black things in your vision which you are describing are called "floaters" in medical terms.
Usually they are harmless but annoying as they obstruct the vision.
However if you have a shower of black dots, flashes of light or you are unable to see clearly in any portion of your vision then you need to consult a doctor.
If you have high minus power then there are chances that there are weak areas in the retina.
Next Steps
Please check your eye power, and get a retinal examination done.