Hey, I did a little research on some binding vows that involve black flash and thought to tell you, just in case it helps.
In fact, you can indeed make sacrifices to buff black flash.
What you can't do is make a binding vow to make yourself land black flashes more frequently, since it's a phenomenon that occurs by pure luck and focus.
But you can alter its effects. You can make the potential boost stack, or you can literally increase the boost it gives per black flash.