In the first place, it should be noted that there are certain requirements for a Muslim woman’s dress. These are:
1. It must cover the whole body.
2. It must not be tight or transparent.
3. It must not delineate the parts of the body, especially those parts that are sexually attractive.
4. It must not be a dress that is usually worn by men.
Jurists, including Imam Ash-Shafi`i, have unanimously agreed that hijab is obligatory upon every woman; however, they differ on covering the face. Some jurists hold that the face must be covered whereas others maintain that it is merely a recommended act.
There is no specific significance for the color black to say that the dress should be black. In some cultures, certain colors are reserved for men; in such a case, a woman should not wear those colors while in that cultural setting.
Elaborating on this issue, Sheikh Muhmmad Saleh Al-Munajjid, a prominent Muslim scholar and Saudi lecturer, issues the following fatwa:
“It is not one of the conditions pertaining to the Muslim woman’s dress that it should be black. A woman may wear whatever she wants, so long as she does not wear a color that is only for men [according to the custom of the country], and she does not wear a garment that is an adornment in itself, that is, decorated and adorned in such a way that it attracts the gaze of men, because of the general meaning of the verse:‘and not to show off their adornment…’ (An-Nur: 31)
“This general meaning includes the outer garment, if it is decorated. Abu Dawud narrated from Abu Hurairah that the Messenger of Allah (peace and blessings be upon him) said: ‘Do not prevent the female slaves of Allah from attending the mosques of Allah, but let them go out unadorned.’”
In the Fatwas of the Standing Committee for Fatwa and Research in Kingdom of Saudi Arabia we read:
“It is not permissible for a woman to go out in a decorated garment that attracts people’s gaze, because this is something that tempts men.
“The dress of the Muslim woman need not only be black. It is permissible for her to wear any color of clothing so long as it covers her `awrah (the parts of the body that have to be covered), does not resemble men’s clothing, and is not so tight as to show the shape of her limbs or so thin as to show what is beneath it, and does not provoke temptation.
“It is not a must for women to wear black. They may wear other colors that are worn only by women, do not attract attention, and do not provoke desire.
“Many women choose to wear black, not because it is obligatory, but because it is farthest removed from being an adornment. There are reports that indicate that the women of the Companions used to wear black. Abu Dawud narrated that Umm Salamah said: ‘When the words ‘and to draw their veils all over juyubihinna (i.e. their bodies, faces, necks and bosoms [one interpretation of the meaning])’ (An-Nur: 31) were revealed, the women of the Ansar (Helpers) went out looking as if there were crows on their heads because of their garments.’
“This is to be understood as meaning that those clothes were black in color.”
Excerpted, with slight modifications, from: http://www.islam-qa.com