Last week i did read an online article (reference not available) mentions the continued use of Dubbin and similar methods for water proofing the boots did turn the leather colour from brown to black.
And as far back as i've been able to find in British military history, when practical and even when not, The British Army has been polishing kit as long as there has been a British Army. So, even those few soldiers who may not have had available a second pair of boots, black, polished for inspections and ceremonial duties, could probably bring up a pair of boots, used in the field/trenches to minimum applicable parade standards in a surprisingly short albeit frantic period of time.

Jacques