Aside from the problem of a black backsplash making the kitchen look too dark and showing spots, I really don't think it is a good choice to go with soapstone. The soapstone will be either a charcoal gray or an almost-black (possibly blue-green-black) when oiled or waxed. Neither of those shades is going to look particularly good with a pure black tile backsplash next to it. Now if you really want a dark backsplash to go with your soapstone, consider a dark gray matte-finish tile. Here's an example:
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black backsplash with accent tile
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