Notice these expressions in the poem and guess their meaning from the context:

rancid breath	squelching tar
spectroscopic flight of fancy
rearing on the thunderclap	brunette
peroxide blonde	clinical assent
raven black

 

rancid breath: Rancid means a matter which is offensive or disagreeable. Thus, the voice in which the lady speaks to the poet is under an immensely nasty or insulting breath.

squelching tar: The verb squelch means to strike or press with crushing force. Thus, the expression used here is that of a huge amount of compressed tar, the dark coloured product obtained after distillation of coal or wood, expressing the complexion of the poet.

spectroscopic flight of fancy: The word spectroscopy originated from the concept of dispersion of visible light into seven different colours. Thus, the word explains the dispersed flow of thoughts of the lady after talking to the erudite poet. Her fancies of a “dark” man gained wings and attained new levels of interpretations when she had to submit to the fact that she knew lesser than the person on the other side of the line.

 rearing on the thunderclap: A thunderclap refers to something resembling the sudden occurrence of a thunder, as in loudness or unexpectedness.

brunette: Brunette here refers to dark hair and, often, dark eyes and darkish or olive skin.

peroxide blonde: This expression refers to a harsh or unnaturally bleached palm and sole of feet rather than a natural fair complexion. Peroxide is a chemical which is used as a bleaching agent.

clinical assent: The voice of the lady in the poem seemed clinical while assenting to the poet's revelation. It refers to the concerned voice based on a vivid and actual observation of the poet, giving in to the situation after a lot of thought and inspection.

raven black: Here, raven black is a metaphorical expression to describe the intensity of the colour black. Raven is supposedly a very large, dark complexioned bird of the crow family. This metaphor is usually used to describe dark-skinned people.