Wikipedia's list of characteristics is actually pretty accurate. An assault rifle is:
- Shoulder fired, as opposed to crew-served
- Capable of selective fire (fully auto or burst, and single shot)
- Intermediate power (cartridge larger than a pistol, but smaller than battle or sniper rifle)
- Detachable magazine-fed, as opposed to belt-fed, or internal magazine
- Capable of effective fire to at least 300m (~330yds)
Most "black rifles" meet all the criteria except #2, which I would consider the most important defining characteristic with respect to lethality and firepower.
Ironically, selective fire is the characteristic that the gun-grabbers completely ignore when "defining" an assault rifle. They'll usually substitute some stupid s**t like a pistol grip or bayonet lug as defining features.