Sine last december, i sometimes see on and off black small floating dots in my vision. they go away after i blink a few times. what causes this?

Floaters: Since they go away with blinking, the dots may be caused by dryness on the surface of your eye. Allergy with tiny amounts of mucus can also cause that symptom. Commonly, thin strands of connective tissue that run through the vitreous jelly in the back of your eye cause these symptoms, but in that situation the dots float away with change in direction of gaze, not so much with blinking.

Floaters float!: Floaters are tiny clumps of gel or cells floating inside the vitreous, casting shadows on the retina. They are more visible when you look at the sky or a bright, evenly lit surface, and may look like little dots, circles, lines, clouds or cobwebs. You have always had some floaters since birth, even if you were not aware of them. New floaters come from tiny bits of retinal tissue.