Hi Natasha,
Black spot that seems initially a good focus, but it dissolves - that sounds like third jhana to me. I get a very similar effect when using the "eyelid colors" as a concentration object for samatha. (With me, first being a steady white spot / nimitta, second a kaleidoscope symmetry of spots and sparkles).
Could be something else, of course. But judging from your thread over at KFDh, 3rd Jhana is accessible to you. Also, the dark night has all these 3rd jhana characteristics, such as not finding a good focus for concentration, for example. Not being able to access review fruitions seems to be a feature of the cycles of higher paths. Is that where you are? Dark night of a big insight cycle?
Have you tried to focus on, or rather, pay attention to, the periphery of the visual field when the black spot turns up and then dissolves? How do the vibrations you mentioned in the other thread figure in the picture at the point when the black spot does its thing? In my last big cycle, grabbing hold of / tuning into the slowly flapping background vibrations did the trick of propelling me into equanimity.
Also, have you tried off-cushion practice? I often (very often during the day) just play around with the locations of the sense of observer, the "spot of attention", and the thing I'm looking at. Just like in that
painting by Mach, in fact.
The neurological condition you mentioned on the other thread, if it is migraine, I remember Duncan Barford from openenlightenment.org / thebaptistshead.co.uk mentioning migraine attacks. Here are two of his blog posts where he mentions attacks and other symptoms:
A False AwakeningEnlightenmentHe's very accessible and thoughtful; maybe you could get something out of contacting him.
Cheers,
Florian