There have been many attempts to define what the essence of black metal is and the definitions that came out of it, were probably as many as the persons participating in such debates.
It is obvious though that black metal is not a genre that can be classified according just to the sound or the way the instruments are played. The lyrical content is the most important factor here. Equally important is the whole atmosphere the music evokes. The image (when there is one) plays a great role in this atmosphere. One could never accept a band wearing baseball caps or budgie pants for instance, claiming to play black metal, regardless their musical abilities or the way the music sounds.I would also go as far as to claim that the image could be almost worthless when talking about a genre that is not "live concert friendly", as black metal is. There are plenty of one man bands or regular bands that choose not to perform live, that the point of taking pictures and releasing them seems to be in vain. I think it would be interesting to mention the example of Deathspell Omega, that have created an image of their own based just on the abcense of an actuall "image".
Would it be "heretical" to claim that the least important factor that defines black metal is the actual "sound"? Such was the situation in mid 80's when genre progenitors like Mercyfull Fate, Bathory, Venom, Celtic Frost, Mayhem or Sarcofago had little in common regarding their sound yet noone thought to distiguish them in the core of their music.
Euronymous used to say that the bands he liked were the ones capable to send chills down his spine upon listening to their music. His wider view on black metal reflects also on Mayhem's music and to his "policy" regarding the bands he sponsored and those he aimed to sign on his record label. Thus he never hid his admiration for the darkness of the southern american bands, released the debut album from Japan's Sigh and also aimed to sign Greece's Rotting Christ or Italy's Monumentum.
It is of course no coincidence that the scene Euronymous created, lost it's orientation and focus after his murder. Inferior successors narrowed the horizons of the whole genre both geographically and musically. The latter would be the unavoidable conclusion of the former, but narrow minded (or no minded) people brought both consequences at once. "De Mysteriis dom Sathanas" acted as the cornerstone for the norwegian black metal foundation but the creation that came out of it would put Euronymous to shame. "De Mysteriis..." is probably the best black metal record of all time and it's misuse by the "true" norwegian black metal clique was a pity. This record alone should be enough to kick start a worldwide tsunami of truly dark and satanic music, yet the norwegians managed to limit its potential to a tiny wave in a (norwegian) lake.
The genre lost the point moving the lyrical content from satanic-theological-spiritual issues to an infertile heathenism, pointless viking worship and regional folklore. Of course the criminal acts that took place offered the utmost assistance in order to draw the people's attention to Norway. They also offered the opportunity to infinite bandwagon jumpers that lacked the talent, potential or true spiritual devotion, to tag themselves as "true", usurp the bm name and create a hype that only led black metal to stagnation. As time is the ultimate judge, the "true norwegian black metal" tag sounds comical today, as the bands that once used it have nothing to do with bm today and tend to denounce their past.
It took almost a decade and the strengthening of the orthodox black metal movement, to put the genre back on track in all aspects (stylistically, spiritually, musically and lyrically). I believe that as long as the same mistakes won't happen again Euronymous' legacy would be much safer this time.