So, to sum up what Dick was saying -- there is no truly pure line of shiro with good sumi anymore because showa took over for a while then shiro with better sumi was re-developed out of showa.
Reminds me of a gorgeous 8 yr old shiro I saw for sale last year. Wondered why it wasn't being kept as breeding material -- the guess from those more experienced than I was that this shiro threw showas in breeding. And probably not very good showas.
Doing the showa/shiro cross could end up with lots of interesting fish. I've no idea how you would do the first culling tho'. Do you cull all the non-black fish first, like for old-style showa? Or do you cull all the solid colors first? Obviously there are some that are right out, but the choices are more...