Matt - it's super simple to pass blue without green. You just need a blue tricolor separation filter like
a 47 or 47B. Zero green gets through. Or you could hypothetically use the blue output on an additive
colorhead, though not many of us own those kind of rigs. The only logistical problem with a deep blue
separation filter would be due to having a hard time seeing the projected image when doing complex
dodging and burning under the blue light. I don't personally have this problem because even my cold
light is quite powerful. I have a 14x14 V54 Aristo which uses a single tube of blue-green, so works
wonderfully for split printing. Without any filtration, it lands VC paper around grade 3.