S. R. Faruqi:

In a blister there's water; and in the heart, blood. In calling the heart a blister there's also the 'implication' that because of sorrow and grief all the blood had turned to water. In a 'single blister' is the suggestion that in the breast there were many blisters, and the heart too was one among them.

In a blister there's no throbbing or beating, but in a heart there's beating. The heart that would have become a blister, hasn't even retained the essential quality of a heart. Thus it's proper only for it to burst and flow away. By saying that the breast was very much beaten, he has also provided a proof/justification [javāz] for the heart's bursting and flowing away.

Zamin Ali Jalal too has well composed the theme of the heart bursting and flowing away like a blister:

kyā jānūñ dil kā ḥāl kih furqat meñ bah gaye
bahutere ābile mire sīne se phūṭ kar

[how would I know the state of the heart? --for in separation, there flowed away
many blisters from my breast, having burst]

In Jalal's case, if there weren't so many words, then the verse would be even better.

FWP:

SETS == EK; GROTESQUERIE
MOTIFS
NAMES
TERMS == PROOF

This is one of only a handful of cases in which SRF has selected for inclusion in SSA, every single verse of a ghazal.

SRF points to the reading of ik as deprecatory: the heart was a 'single' or 'particular' blister among many. Jalal's verse is even more explicitly dismissive. But of course, ek can also have a sense of 'unique' or 'excellent' (see the definition above). Or in this case, the idea that the whole heart was one single blister can have an emphatic dignity of its own.

But still, it's a dignity that's also always on the verge of grotesquerie. For ābilah can refer to a 'pustule' of the kind that develops in smallpox (see the definition above). And the idea of a 'pustule' bursting open-- well, we have to try hard to avoid thinking of pus flowing out of it. And really, the ghazal world isn't hospitable to truly disgusting imagery.

Note for grammar fans: The apparent parallelism between phūṭā gayā and kūṭā gayā is deceptive; the latter is a transitive verb in the standard perfect passive form, while the former is an intransitive verb and so basically (except in rare and special cases) cannot be made into a passive. Thus we should read phūṭā gayā as two perfect verbs in sequence ('burst, went'). It would also be possible to read it as short for the adjectival perfect participle followed by a perfect verb: phūṭā huʾā gayā ('in a state of having burst, went'), but in a contextual sequence like this, the two-perfect-verbs reading seems more attractive. The same two-perfect-verbs reading applies to chhūṭā gayā in {52,2}, and ṭūṭā gayā in {52,3}, and chhūṭā gayā in {52,5}. Alternatively, we could treat them as archaic versions of the passives chhoṛā gayā and toṛā gayā , which would solve the problem.