S. R. Faruqi:

dam bharnaa = to call out

It's not clear why from a single cry of huu , the mirror of the world will become black, or what is meant by the mirror of the world. Thus there are a number of possibilities.

The world is a mirror because in it the Absolute Reality is reflected. That is, the world is the kind of mirror in which fixed laws are apparent. Or the world is a mirror because just as no reflection remains in a mirror, in the same way here too nothing remains. Or again, creation is a mirror-chamber, and in it a single mirror is our world.

A qalandar is a person who is free of worldly attachments; thus in his view the world has no duration; his heart is given to Absolute Reality. One name for Absolute Reality is huu ; that is, the radiance of that one in the state of Absolute Existence. The point of this is that mankind is devoid of right/truth, thus he gives to the right/truth the name huu , which is to say 'that one' [vuh]; he doesn't give it the name 'we' or 'I'.

Before Absolute Reality, all signs are petty and false. Thus when the qalandar, by saying huu , calls out to the Absolute Essence, or testifies to its truth, then for the petty and false signs of the world to attain oblivion or become black-- that is, for their not to be visible-- is a natural result.

For a mirror to become black means that in it nothing would be visible-- that is, for reflections and illusions to be destroyed. Thus the interpretation is that if some existence about to attain union with the Absolute Essence, or some person free of worldly attachments, with a sincere heart would call out to the Absolute Presence, then all these artificial and illusory affairs would lose their status and value. The qalandar by his deed can prove that the world is without reality, that nothing is real apart from God.

Dard too has composed this theme in a very powerful, but slightly obvious, way:

mi;T jaa))e;N ek aan me;N ka;srat numaa))iyaa;N
ham aa))ine ke saamne jab aa ke huu kare;N

[they would be erased in a single moment, all the multiplicity of manifestations
when we would come before the mirror and say huu]

In Mir's verse, saying 'the mirror is black' has much more power than saying 'the mirror would be black'. There's immediacy in it, while in 'would be' there's only possibility and futurity.

Nisar Ahmad Faruqi says that dam bharnaa means 'recitation in the heart [;zikr-e qalbii], although in fact recitation in the heart is referred to as paas-e anfaas and hosh dar dam . But the late Nisar Ahmad Faruqi made this fine point: that in recitation in the heart 'a stage is also reached when the recitation too is obliterated, and only the Recited remains; and this is the state where the Pure Essence in totality makes itself seen. The Sufis say that when the Pure Essence is seen, there is darkness upon darkness; that is, all visible things are obliterated.'

[See also {1374,7}; {1896,9}.]

FWP:

SETS == GRANDIOSITY
MOTIFS == ISLAMIC; MIRROR
NAMES
TERMS == INSHA'IYAH; REFRAIN

SRF's point about the dramatic immediacy of saying 'the mirror is black' rather than 'the mirror would be black' is a fine one. It is an intriguing case in which insha'iyah speech incorporates a (seemingly) ;xabariyah statement that only enhances the insha'iyah effect. And further to enhance that effect, in this verse the refrain is beautifully punchy and apropos. It wouldn't take long to say a single huu , but it would take even less time for the power thus unleashed to turn the whole 'mirror of the world' into blackness. The huu itself is the Arabic third person singular, and is used by Sufis to refer to God (as 'He' often is in English).

Consider this verse in the light of Mir's other ke biich verses: in most of them the refrain feels neutral, and could be replaced by me;N without making much difference, while in some it feels like an inferior, obligatory substitute for me;N (such that part of the pleasure of the ghazal is seeing in how many ways the poet can manage to wrestle his refrain into submission). But here, the refrain shows itself to brilliant advantage.