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Yeah you'd think so but no. Yellow coticule slurry is yellow, but Blue whetstone slurry is not blue :)
Purple / maroon makes that a bleu / yellow combination stone.
They're not exactly rare, but...

Also, try lapping the dark side. If the slurry is black or dark grey, the backing is worthless slate.
But if the slurry is milky brown / purple-ish, it's blue whetstone. In the past they used to...