So many comments! :-) Thank you all, from the heart. I'm glad that you appreciated my tribute to the unbeatable Bad Brains. I'd like to respond now to some of your thoughts.
@ Gnagnera: yes, NY's hc-metal came a few years later with bands like Agnostic Front or the amazing and eclectic Ludichrist, but in general, the kids from NY often cited Bad Brains' records as a source of inspiration... regarding the precursors of hc, it would take an eternal discussion: what you say about DOA is true, and also about Middle Class (it's a shame they disappeared so quickly)... then there are many isolated tracks that are "unintentional" precursors of hc (I think of "I See" by the Testors, a one-and-a-half minute blistering track)... the DK were icons of hc, very influential, but they also had a peculiar style influenced by the inclination towards the corrosive satire of their lyrics...
@ Alex: to hear the blues scales in Black Dots, you have to do the opposite of Pinhead: play the record at one-tenth the speed :-D
@ Bore: but you see, the concerts at CBGB were calm compared to typical hc shows of the time :-)))... CBGB was a new wave venue, and the 14-year-old kids of the time considered new wave to be music for old folks: when they played in places like that, people were shocked! broken bones everywhere!!!
@ JDV: basically in the thrash metal riff you should hear all the notes... the execution is more precise and the songs are more intricate and powerful, with two guitars, double bass, solos, etc... hc generally is more "handcrafted," more anarchic and spontaneous, the sound is dirtier, chaotic, the rhythm is frantic... the vocal approach also changes... try comparing Motorbreath by Metallica with Filler by Minor Threat...
@ Jack: I've always thought that rock'n'roll was musically more black than white... it's true that the country-folk tradition also had its weight on the giants of rock, take Neil Young, for example... more than anything, I meant to highlight the exquisitely rhythmic element, which was brought into MODERN popular Western music precisely by black people...(not that tarantella or polka were rhythmless music, to be clear ;-D)
@ Kosmo: thanks for the links! :-)