S. R. Faruqi:

tabḳhāl = blister

This verse has no special excellence, but it can be placed among the especially prime examples of 'thought-binding'. By 'thought-binding' is meant that the theme would be a very far-off one, and half the time there would be no special beauty in it, but it would have uniqueness/oddness [nudrat]. In a 'thought-binding' type of verse the 'proof' is usually weak, but it's also not such that the verse would be entirely without a 'proof'.

In the present verse, a proof is included, since he calls the beloved's lips 'the water of eternal life'; however, in 'water' the existence of a pearl is not a guaranteed outcome. The wordplay of 'ruby' and 'pearl', and in the first line the alliteration of 'l' sounds, create additional excellence in the verse.

On the theme of 'a blister', if you want to see a verse of pure 'thought-binding', then look at Ghalib's [unpublished verse] G{236x,4}:

āyā nah bayābān-e t̤alab gām-e zabāñ tak
tabḳhālah-e lab ho nah sakā ābilah-e pā

[the desert of seeking did not come as far as the footstep of the tongue
the blister on the lip was not able to become a blister on the foot]

Ghalib has mentioned a blister not on the beloved's lip, but on the lover's lip. Nasikh uses a theme of the beloved's being pitted with smallpox:

ābile chechak ke jab nikle ʿiżār-e yār par
bulbuloñ ko barg-e gul par shub'hah-e shabnam huʾā

[when the blisters of smallpox emerged on the beloved's cheek
the nightingales thought they were dew on a rose-leaf]

As is clear from Nasikh's and Ghalib's verses, in 'thought-binding' the pleasure of meaning is small. For this reason Ghalib himself said about one of his own verses, 'in this verse the thought is very subtle, but there's no pleasure at all-- that is, 'to dig up a mountain, and bring forth a straw.'' [[This is a slightly altered form of the passage translated in G{28,1}]].

FWP:

SETS == GROTESQUERIE
MOTIFS
NAMES
TERMS == PROOF; 'THOUGHT-BINDING'

Ugh! This is a fairly grotesque image. It's not that on the ruby lip of the beloved there's a blister-- not at all. Instead, in the wetness of the 'water of life' provided by the beloved's lips, there's a 'pearl' (spherical, light in color, maybe a bit glistening, emerging from water). And if the speaker 'obtains' this water from the beloved's lips, the suggestion is surely of a kiss. (Does he somehow 'obtain' the blister-'pearl' too?) Really, blisters just don't lend themselves to being poeticized.