Most immediately, the name Khorasan or Khurasan seems to come from the fact that several of the leaders of the group in Syria were chosen by al-Qa‘ida leader Ayman al-Zawahiri from leaders of the movement who had been inside Iran (only sometimes tolerated by the Shi‘ite regime) or in Afghanistan and the Pakistani borderlands. They had links to al-Qa‘ida's council for affairs in the Afghan region, the Shura of al-Qa‘ida in the land of Khurasan or the Khurasan Shura. But the choice of "Khurasan" has a deeper resonance.
There is a hadith or tradition of the Prophet Muhammad which most hadith commentators consider weak and probably fabricated,that with some variations usually is cited as something like this:
If you see the black banners coming from the direction of Khurasan, then go to them, even if you have to crawl, because among them will be Allah’s Caliph the Mahdi.Indeed, as I said, most commentators do not accept this as a real tradition of the Prophet, and many commentators and historians assume it was created to garner support for the ‘Abbasid Revolution which overthrew the Umayyad dynasty in AH 132/AD 750, and which started in Khurasan with a revolt led by a somewhat mysterious figure named Abu Muslim, and whose emblem was indeed a black banner. Some versions of the hadith even include phrases like "they are the sons of al-‘Abbas." The third ‘Abbasid Caliph (and the father of Harun al-Rashid) took the name al-Mahdi.
|Jabhat al-Nusra Flsg|
And as an aside, some versions of the hadith suggest that the individual fighters in the armies advancing under those black banners will be known not by their given names but by a kunya (the "Father of" name) followed by the nisba name formed from their home town or country. Hence the standard form of the jihadi nom de guerre: Abu Bakr al-Baghdadi, Abu Mus‘ab al-Zarqawi, Abu Mus‘ab al-Suri, Abu Anas al-Libi, and so on.