Stefan emailed me saying he had some 4001 Blue-Black and wanted to compare it to Tanzanite to see how suitable a replacement it really is. I sent him some samples of the ink and he wrote an incredibly thorough and comprehensive review/comparison of the two inks.
Here's a preview of his post, to see the whole thing visit Stefan's blog, here.
Long story short, the inks are pretty darn similar. Not exactly the same, but very similar. See how Stefan gets to this conclusion here. Thanks so much for all of the hard work, Stefan!