I assume the pircture of Jimi Hendrix is a reference to the fashion for wearing C19th uniform tunics and other 'Victoriana' that grew up circa 1966, at the height of the 'Swinging London' era. There was a shop called 'I was Lord Kitchener's valet' that specialised in vintage uniforms. The fashion inspired the title and the cover of the Beatles 'Sergeant Pepper's Lonely Hearts Club Band' LP. Collectors today would probably weep at the fate of some antique items.
As for black soldiers at NAM, in a refit in the late 1990s (IIRC), a number of realistic mannequin figures, portraying British soldiers throughout history, were posted throught out the main galleries. For 'The Road to Waterloo' exhibition, there was a figure depicting George Rose of the 73rd Regiment, sitting exhausted by the side of the road. Rose has been, a little quaintly, described as 'The Black Watch's black soldier.' I vaguely remember the West India Regiment figure in a case in the main gallery.
I think "the first official representation of black soldiers in the army..." refers to the earliest known contemporary depiction of black troops being recruited, albeit in dedicated regiments for Caribean service, i.e.a poster. The fact that men like Rose were scattered throughout Line infantry regiments was either not acknowledged at the time or possibly, not thought of as particularly remarkable.
Only in later C19th depictions was the presence of black soldiers probably found inconvenient, although in the famous canvas of Nelson being mortally wounded at Trafalgar, a black seaman is shown on the deck of HMS Victory.
PS, from our sister forum:
Black Soldiery in the Peninsular Armyhttp://www.napoleonicwarsforum.com/view ... =rose+73rd