Where Every Scroll is a New Adventure
I looked into it this morning and Midnight Blue truly fits Dazai!
When Dazai was in the port mafia ('a limited spectra') he appeared black morally, and in terms of color, a bringer of death and highly intelligent. But when Odasaku displayed a more realistic gaze on him, due to Odasaku being an as-it-is person, we discover he was just trying to get close the the human nature, and see if it would give him a will to live.
Midnight Blue is composed of black and blue, the black gives it it's dark shade. In the eye of "sunlight" otherwise referenced to as the ADA, we can see the brighter aspects of him.
I think that's what Odasaku saw in him too, his blueness.
Bungou Stray Dogs character colors From the 2nd BSD Guidebook
All of their names are on the left side, with their corresponding color on the right side.