S. R. Faruqi:

For the image of a painful boil/blister, see

{64,7}.

There he has given for the heart the simile of a 'ripened boil/blister'; but here, by saying 'would keep ripening' he has expressed a continuous process. In a ripening boil/blister there is a restlessness and burning that keeps on increasing. In the outward form of the boil/blister, there's no special alteration. In this respect, to call the heart hidden within the breast a 'ripening boil/blister' is fine. He has not made clear what the state of the heart had become after the night passed-- perhaps it turned entirely to blood, or perhaps we ourself were finished off.

By depicting the heart as an agent (the heart gave pains) he has created a new theme: that the heart in its place was a free agent, and was giving us pain and trouble. This idea is also correct, because when is the lover's heart within his control? As far as I know, after Ghalib, only Ibn-e Insha, following Mir, has versified the image of a ripened or ripening boil/blister. Ibn-e Insha's effort is worthy of praise, but through lack of skill/dexterity he has composed an entirely uncouth/ugly [bhoñḍā] verse: The image has been destroyed, and the meaning too:

yih dil hai kih jalte sīne meñ ik dard kā phoṛā alhaṛ sā
nā gupt rahe nā phūṭ bahe koʾī marham koʾī nashtar ho

[this is the kind of heart that is, in the burning brease, a clumsy/awkward boil/blister of pain
it would neither remain hidden, nor burst and flow away-- if there would be some salve, some lancet!]

[See also {64,11}.]

FWP:

SETS == GROTESQUERIE; MUSHAIRAH
MOTIFS
NAMES
TERMS

Here's another example of what I call 'grotesquerie', in which the 'ugh!' reaction tends to overpower one's appreciation of the verse. There's a kind of excruciating, too-physical, too-powerful effect of the second line that makes the imagination flinch and recoil. Of course, this is a subjective reaction, and I'm only describing it as such. Many nowadays may feel it, but did the original audience?

Whatever they may have felt, it's clear that Mir expected their reaction to carry the whole verse. The whole second line lingers on the preparation for this one image, and then the phoṛā finally bursts (so to speak) upon us at the last possible moment, in classic 'mushairah-verse' style.

Note for translation fans: How to capture the pluralizing and variegating effect of kyā kyā ? The good old southernism of 'what-all', a less common parallel to the similarly pluralizing 'y'all' (for 'you-all' of course) seems to be the best available solution.