If you've got one, I'd try a blue Sharpie on that centre line. I bought a pack of coloured Sharpies and I've used them a lot for small touch-up jobs.
I'd try and feather the edges of the darkburst a bit more if you can. In some areas it blends, but in others there's more of a solid line. You could try stippling the dark colour on with an almost dry coarse textured rag.