It's all the time for me in the right lighting anytime I blink or move my eyes the black spot/shape thing appears. However, my mother also can see one - I've asked her enough vague questions and she's described the exact shape, movement, characteristics of the black shape. The only difference between me and her is that she doesn't worry about hers!
Flashbulb mostly happens when I blink or turn my head quickly towards the sky or a bright window, like the white of the net curtain. I see those sparkles on the white curtain too and the area of my vision with the flashbulb is very sparkly after I notice it.
This all started around the same time I got visual snow (which I think was probably caused from years and years of anxiety attacks and worrying but not sure).
My eyes were checked by an ophthalmologist at the hospital a couple weeks ago, but I've been noticing these issues since April. The doctor didn't know offer any theories and was her idea to order the scan (although I wanted one too) though she said if they suspected anything like a tumour I'd be having an urgent scan in the next week or so. I'm also going to have an electroretinography.
The only things that have reassured me slightly are 1) my mother sees all the same things as I do, and seems to have the visual snow issues too, which makes me think it's all connected to that 2) a lot of people with visual snow and these symptoms come back with clean MRIs.