You need to do a little more research! if you do black and sand back the grain will be black!
I did say 'at a guess' because the resolution's so poor........it's defo doesn't look white filled - your clear filling sounds a reasonable analogy and quite a neat trick. Errr....just in case i missing something, the end grain, i found when i did a guitar out of ash, was harder to sand flat than the non end grain (if that makes any sense) so your statement would be back to front.
Jem