Flat black may have been a safeguard measure if the darkroom was also used for exposure and testing of total darkness materials (duplicating, copying etc) as Kodak recommends an all black darkroom for these purposes. But for a darkroom in which safelighting is acceptable Kodak recommends flat white for the ceiling. For the walls Kodak recommends white, cream, other light colours or a colour similar to that emitted by the safelight, while flat black is recommended for the wall directly behind the enlarger (or other panels nearby), and around any light locks. These recommendations were in Kodak pub. K-13 (Photolab Design) and are still in K-4 ("How Safe is Your Safelight?") under Placement of Safelights.
Originally Posted by Prof_Pixel
Interestingly, in darkrooms where safelighting is used Kodak actually advises against painting the darkroom black. The explanation is that safelights will appear as "pools of light" on the dark background which apparently can cause eye fatigue.