There is a progression from the actual newspaper picture to the Great Tay Bridge disaster and finally the consequences. To maintain this narrative, the language has to work hard, defining themes and flow.
The repetition of "one black spot" defines two of the main themes - spots and blackness. The choice of these two themes evolves naturally from the double meaning of the phrase - the black mark on a supposedly spotless career , and the spots that make up a newspaper picture. There are other meanings as well - the black gap in the collapsed bridge, and the specks of soot and printers ink make a pivotal point in the poem.
Other themes are present in the poem. The "ordained plan" casts a shadow over the events, and the Tay Bridge disaster could be seen as preordained because of the poor construction techniques used in building the bridge. The epigraph is not just ornamental, but fits into the theme of pre-ordination, with McGonnagal's moralising echoing the Victorian religious ethos of "pride comes before a fall".
The transitory nature of human achievement is another theme - "like his famous bridge, the glorious knight falls quickly" and "constructed glory smudges away" like newspaper ink on the fingers of the readers. The final note of explanation also concentrates on Bouch's success bringing a note of irony and perhaps a more sympathetic look at his career. The reader has to research to link the poem to the historical event which is not mentioned at all by name in the poem. Well, not everything can be made easy!
The title of the poem indicates its primary subject is Bouch and the picture, not the Tay Bridge disaster. The poem concentrates on the picture, its construction and printing. It also concentrates on the effects of the disaster on Bouch - the "bloodshot eye", the grey face, the grief and shame, which eventually leads to his death. The pivotal point in the poem that links these two topics is the phrase "the locomotive sound of printing press". Here the language is working hard, and again in the "ink black gap", a hard sounding little phrase bringing the disaster back to the photograph, and the wihrlpools which immediately follow, moving on to Bouch.
More details and the photograph that inspired the poem can be found at http://www.kosmoid.net/technology/bouch .