Black can be an arse. There's not only "what to use to highlight it", but "how much highlighting can I get away with before it no longer looks black".
The "what colour to use" has been talked about on various threads before. I don't recall the extreme highlighting issue coming up so much.
It's something, despite knowing the problems of over-highlighting, a trap I fell into. It's all too easy to just approach black and start painting as one would anything else. But where anything else requires a range of tones going through the Shading/Basecoat/Highlights, black need to be more present in the Basecoat stage and absolute in the Shading range, else it can end up too light and instead of looking black it could end up looking like a dark grey or dark blue or something.
You could just approach it with extreme highlights (of whatever easy and convenient paint, Codex Grey, a Blue-Grey, a Dark Turquoise, whatever) and apply that as edge highlighting. That is, using the side of the brush, pick out the edges. From there you could add white to your paint for further high contrast highlights on the uppermost edges.
To get that first highlight tracking back into the black, you could mix whatever you used with some black, thin it down, and paint along the line, wiggling the brush some helps breaks up the blockyness, blending it together. Can always dilute some Black Ink Wash (or just the apparntly pre-thinned weak GW Badab Black) to serve as a bit of a glaze. Not applied as a wash, dolloped on, but with a small amount on the brush and carefully applied it can tint what's already on the mini without dominating it too much. That that bind together the stages that have already been applied.