When the kernel is booted with vga=normal (or no vga=) and quiet on i386/x86-64, the screen goes white, and is replaced with white-on-black characters as they are written, eventually pushing the white screen away.  A screen clear resets the display to a black screen.

Any other combination (vga=792 and quiet, or vga=normal and no quiet, for example) produces the expected result (black screen from the beginning).

Reduced to defconfig and is occurring as of HEAD.  It's not a new problem though, occurring as far back as 3.0.