Let's test your slurry stone first: put some water on the dark side and rub with a little piece of sandpaper. Is the resulting slurry of a dull gray color? Than it's slate, and no good for anything else that reinforcement of the Coticule side. Does the slurry has an obvious purplish hue? Than it's a BBW.
If it's not a BBW, I wouldn't use it for creating slurry, though I've never tried. What I however did try is to use a Coticule slurry stone on the BBW, and it works just perfect, with only one disadvantage: normally a BBW-slurry has less of a limiting factor on the maximum keenness than a Coticule-slurry. But in most cases, you're going to apply some sort of dilution strategy anyway, in which case it doesn't matter at all. We are currently testing 2 methods for honing razors with just a BBW, and with these methods, it doesn't seem to matter at all wheter a Coticule or a BBW slurry stone is used.
For the rest, I second Phil's and Rico's advice: if you don't trust it, go with a diamond hone, or a piece of good quality wet&dry sandpaper.
Kind regards,
Bart.