I've been out of that business, for a while, but back when I worked on them, there wasn't a testing place for 3126 injectors.

 If we saw, Black fuel, all 6 injectors got replaced.

I'm unsure "How" they would test for internally leakage, in a short testing period; BUT I've never even seen any of their testing equipment, for 3126B injectors. {Would they have a clear sight glass, on the return fuel, leaving the injector, on a test stand, then recycle that same fuel, while testing and after 10 minutes, see the color difference, if any . = Heck, I don't know}