hypothesis

I cannot speak with assurance on British or Canadian dyed uniform coats, but the following situations have resulted in US uniforms being dyed black in the first half of the 20th Century:

1. Post war civilian use, as you noted. Often by the vet himself.

2. Replacement issue to enemy POWs. You can't easily "undye" black. . . (Not a brilliant idea, as you've just given the prisoners access to cloth that can be retailored to pass as common civlian attire.)

3. Surplus issued to civil prisoners (uncommon).

4. Issue to occupation government employees. After the war is over and teh army mostly demobilized, it's cheaper to dye "excess to needs" uniforms (especially "obsolescent" patterns if you still have them in store) black for issue to your work crews. THEY don't generally have access to plentiful supplies of new clothes due to the fact that you've just finished smashing their economy and infrastructure, service uniforms are durable and already paid for, black dye is cheap (and see #2. . . you've effectively eliminated them from being able to pass themselves as one of the occupation soldiers), and "food and work clothes" are often the BIGGEST part of teh recruiting incentive.

Rick