[GeoffR]

The black flash is the page refresh, it should normally occur only once every 6 pages if you have that set in the Reading Settings.

However when reading a kepub book with the current firmware, if a page has a full-screen image then a refresh always occurs, so if you have a number of pages with nothing but images then you would see it refreshing every time you turn the page.

Edit: I should add that unlike text, images are displayed in two steps: first the image is drawn in black-and-white; then the shades of grey are filled in. For images which have a very light colour palette this two-step process can also look a bit like a flash, although different to the flash of a screen refresh.