If the paint was thin or just covering (hideing) on the first job, and you are blending, you can`t use a sealer at the the edge of the blend. Sand beyound the repair a foot or so with 1500 grit wet, just lightly. Or if you have allready color sanded there , just clean the polishing compounds of,spray the first coat to cover the repair, then each coat after that go a little further, right on top of the clear. you should have 3 coats or what it takes to hide good, the last coat being thin at the blend.when you do the clear do it the same way going even further out.you can also get a blending additive that helps the edge.
Probably what is making it darker is the first paint was not heavy enough to get the right shade of the color,then you put 2 or 3 coats on top of that and actually got the right shade of the color.
This may sound confusing to someone who is not a painter.
metallic is the hardest paint to blend that there is. I try to paint to a seam or panel edge if I can.
When I think I have enough paint on ,say 3 coats I put one more on, just to be sure. HTH