It’s exactly as you’re thinking. I think this じゃないの is simply the same as じゃん, じゃないか or simply じゃない (although that one might be ambiguous sometimes), which can be used to create something like a tag question. It can be used either as a tag question, but you’ll also often see it used to simply express surprise, which is what I think is happening here. So she’s basically stating that she’s surprised by the fact that Tanjiro’s face is pitch-black. 
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