Someone else might have already come upon this, but a recent experiment with Birchwood Casey liquid gun blue solutions, "Super Blue" and "Perma Blue," have shown that both are good alternatives to Blacken-It for coloring Fruil metal tracks.
Rather than re-post and repeat all of the details, here's a link to a thread that I just started on Armorama about it:
I can't imagine that I'm the first person to ever hit on this, so the credit may well belong to someone else. But if you're lookig for an easy to find and cheaper alternative (at least in the US) to Blacken-It, this stuff works! It's even available at my local Walmart!