Yes, "black guns" look mean. So do chopped, ape hanger, no muffler Harleys. So do lots of muscle cars. Lots of tattoos intentionally look mean. The list could go on.
As an owner of one full sized AR style gun, and one pistol style black .223, I can say that like a lot of "hot/mean" looking objects, these guns are both fun and functional. They are the most popular for a reason ... they are light, accurate, reliable, and maybe most importantly, readily customizeable. They are the "Barbie of the gun world" ... only they are real, and they really work. WELL.
The linked is worth a read.
The AR-15’s versatility and adaptability has made it the rifle of America’s militia, which is nothing more or less than America’s responsible gun owners. It is within the AR-15 cult that gun owners are likeliest to get the best education in gun safety, the best training for being a responsible gun owner at home, traveling, or on a range. It’s there that they may get the best understanding of where the gun fits into America’s tradition of republicanism. A country of determined men who have arms like the AR-15, or even significantly less-capable rifles, is almost impossible to rule without consent. Just take a look over at Afghanistan.
ARs, shotguns, and pistols are the core of America's "gun culture". In order to shoot, most serious gun owners are members of a gun club, which is a social activity.
One of the fastest growing shooting sports is Practical Shooting, a challenging and fun activity where you will really learn how to safely handle a gun around others.
American gun ownership is somewhat like golf ... although the adult beverages have to wait until after the shooting is done!