The black flash is, on some readers, optional. What it does is ensure that the screen is 100.00% wiped before rendering the next page. Some readers let you turn it off completely or have it happen only ever nth page that you choose.
My reader allows this but affer experimenting with it I went back to a full flash after every page turn because: 1) my reader is fast enough it makes no big difference and 2) I'm used to the full page flash and the fade-in of a new page without it is jarring to me.
Basically, the black flash is something most of us get used to after a few dozen pages of reading a good book; once you're immersed in the story you no more notice the black flash than you notice the page turn press. It quickly drops out of your perception unless you stop to think about it. (The brain is a very interesting computer.)
That said, if you're looking for pdf reading your choices range from decent (Kindle DX) to tolerable or worse (everything else).
One more time:
PDFs really only look good on a printout or a good-sized PC screen.
Also, PDFs are *not* ebooks. They are digital microfiche. Pretend paper. Electronic documents. Call them what you will. Just not ebooks.
If you *must* do PDF on eink, Kindle DX is probably good enough; just don't expect Nirvana.