The question/claim: "To cover means the black chador (or a specific cut/face-veil); the chador is obligatory." Is this the Qur'an's command, or is the form cultural?
What does the Qur'an say? — the principle is commanded, not a form
The Qur'an sets covering as a principle; it specifies no particular colour or cut:
"…let them draw their head-coverings (khumur) over their necklines/bosoms…" (24:31) — the command is to cover; no colour/shape.
"…let them draw their outer garments (jalābīb) over themselves (outdoors); that is more fitting that they be recognised and not harmed." (33:59) — the aim is recognition/protection, not a uniform.
"O children of Adam! We have sent down to you clothing to cover your nakedness and as adornment; but the garment of God-consciousness (taqwā) is best…" (7:26) — what matters is taqwā and modesty, not the cloth's colour.
Principle ≠ form
- What the Qur'an sets is the principle of modesty. Its form (colour, cut, chador/coat/shawl) is a cultural choice that varies by era and place: in the Arabian desert, in Anatolia, in Iran, in Indonesia, the forms of covering differ — all keeping the same principle.
- "Black" colour and the "chador" cut are the tradition of a specific region/period; the Qur'an does not name them.
An honest limit
- In the text: the principle of modesty and covering the bosom is an explicit command (see "Is the headscarf obligatory?").
- Interpretation: covering the face/hands (the niqāb) and the limits of covering are themselves contested.
- Cultural: "the only valid form is the black chador" puts the form in place of the religion; this is cultural/local.
Conclusion: the Qur'an commands the principle of modesty; a specific colour and cut is cultural-geographic. Taking the black chador to be the only "religion" narrows the principle into a form.
Source: Qur'anic verses (M. Okuyan meal). Built on the distinction between the principle of modesty and a specific form/colour; see also "Is the headscarf obligatory?". Not a fiqh fatwa.