Black Week refers to the week of Sunday 10 December – Sunday 17 December 1899 during the Second Boer War, when the British Army suffered three devastating defeats by the Boer Republics at the battles of Stormberg on Sunday 10 December, Magersfontein on Monday 11 December and Colenso on Friday 15 December 1899. In total, 2,776 British soldiers were killed, wounded and captured during this period. . The events were an eye opener for the government and troops, who had thought that the war could be won very easily. British units were armed with then-modern magazine-fed small arms, the .
https://en.wikipedia.org/wiki/Black_Week